Read the in-game description would be my first answer, but for an actual elaborate one from a 40k lore nerd:

1)No, he's not astartes. Crusaders are former militant clerics of the Adeptus Ministorum who acted as either bodyguards of high ranking priests, inquisitors or warriors of the Imperial church on the battlefield (thought that last use isn't stricly speaking legal and it's why crusaders are a small bodyguard corps and not an army, that's what the Sororitas is for).

2)No, he's not grey knight, probably no on the nemesis weapons since crusader isn't a psyker either and those only work for psykers (force weapons). Allthough one clas is a psyker (not in game yet) so maybe they could get force weapons, but I doubt nemesis ones, since those are grey knight only. Theoreticaly a psyker inquisitor could have one made for his size. And also definetly No on the Termie armor, since this is not an astartes here.

3)Crusaders use power armor designed and build to be worn by average humans. More advanced in manufacture and more difficult to make than standard power armor due to the fact average human's don't have the black carapace to interface with it directly, but doable for special very important people in the imperium like inquisitors and the forces of the Adepta Sororitas

4)No ideea if they plan to add them (not a dev obviousy) but your argument is flawed. The Grey Knights are the Chamber Militant of the Ordo Maleus of the inquisiton, one of 3. They are not strictly speaking the army of the Inquisition since the Inquisition doesn't have an army officialy, just military branches seconded to it's ordos. Btw this includes the Deathwatch for the Ordo Xenos and the Adepta Sororitas for the Ordo Hereticus. The only thing that would qualify as an actual Inquisition army for all Ordos is the Tempestus Scions of the Inquisition, also known as the Stormtroopers. But even they are a formation shared with the Astra Militarum.

You're welcome mate.

Eh, technicaly speaking an inquisitor with experience and gear taking on an astartes is possible. A dreadnought or Decimator demon engine though, that's harder to believe. And I'm going to have to asume we'll get greater demons at some point, though again there have been cases of average joe's taking on these foes.

From a set-up perspective the game is ment to be played in 4-player coop to justify taking on those threats as an inquisitorial strike force. But you can't exactly force people who want to solo into that. I'm curios how this will evolve with time as the game moves through the alpha and the other classes get added, especialy from a single player perspective because as it stands right now, yes, chaos marines and dreadnoughts are incredibly weak.

Then again, the weapons themselves don't work as they should since aparently I can kill a dreadnought with auto-pistols and a plasma cannon, a middleground anti-armor weapon not really designed to punch though the heaviest of plates is better at anti-armor than a melta, which in universes vaporises tanks but in game takes ages to kill even an average human foe. So the balance isn't exactly great and we are missing some weapon options (autocannons & lasscannons for example).

I think from gaming experience the enemy health pools are adequate given the fact all current missions are labelled (for me at least) as having a difficulty of 1. Compare this to the average ARPG when an enemy will die to somewhere between 1-3 attacks depending on strength and excluding larger mobs and bosses. When it comes to making a 40k game into an ARPG I think you have to allow for some bending of the rules in terms of what a single inquisitor is capable of, otherwise we would have a really really dull game. There is also going to be other factors in the game which determine difficulty and built into the mission generators. So i'm sure if you want to play a mission against enemies who are near impossible to kill then you will have the option to!

and just to pick up on the weapon system making sense. I have to agree that while an auto pistol in theory "shouldn't" dent a dreadnaught, you couldn't have a game with this weapon system where you "Had" to have an anti vehicle weapon on you. Maybe in a squad based game yes, or maybe if your character could carry 4-5-6 weapons. But with their current system I think it's just a given that every weapon will need to damage every enemy, with some modifiers of 50% +/- . I do agree with your logic but you just can't make an arpg out of a tabletop simulator. 

A good read on the crusader would be the Soul drinker omnibus, in that book a crusader is sent as judge, jury and executioner with an inquisitor to place judgement on the Soul drinker chapter, So a Crusader is capable of taking out space marines.

Many upvotes. Need more lore-concious people about to explain why things are the way they are so the game stays true to the IP and doesnt get swayed in certain directions by the community.

Porbably the most outdated book you could come up with ;-)

As far as i am aware most of the stuff that's written there isn't even part of the fluff anymore.

Old? Yes. But still one of the better ones. Altough I can't remember that there was a crusader in the Inquisitoral Fleet after the Soul Spear was stolen from the Mechanicus. Long time that i read those books. And I'm still confused about the conclusion in "Phalanx" that the Soul Drinkers are no offspring from the Imperial Fist. I just read the "Beast arises" series and there, just one millenia after Horus, the Soul Drinkers were part in Dorn's Last Wall Protocol. Might be the "Lucky Mech Bonin" Syndrom. Dying in one book and mysterically resurrect in a few books later.. But I'm disgressing :-)

About the Warhammer lore. As Airsick Hydra stated, the game must work for solo as well as for co op gaming. So bending the rules at some point is inevitable. And I hope the Loremasters will aprove most of it. I like the Warhammer universe as complex as it is. But for me the priority for the game should not be to stick exactly to the lore but to catch the overall atmosphere of Warhammer 40K. Dirt, chaos, blood, violence, madness. So if the lore states, that a crusader might have no chance to beat a Dreadnought in single combat... Well as long as there is no unicorn, I can life with that small transgression. In BL novellas the story also always depend from which side it is told. In one story a "hero" dreadnought is almost unstoppable and mows down dozens Chaos Marines without taking critical damage. In another a female (Callidus??) assassin slew a chaos dreadnought single handed. If I remember correctly she did it by destroying some sort of heat exchanger. 

So yes, destroying a dreadnought might be possible for a cybernatic upgraded human ;-D

Officio Assasinorum operatives are as post-human as Adeptus Astartes, they are just hyper-specialized. Callidus, Culexus, Vindicare and Eversor are all not typically battlefield operatives and have very niche modus operandi. I do love the reference to the Venenum temple in the skill trees too... 

The game will have to bend things to ensure those things that are the face of 40k - Dreads, CSMs, Plaguebearers, Nurglings, Greater Demons, and the like - can feature. 

As much as I would be happy with a neck-bearding Inquisition RPG focused on the back alleys of 40k, grimey knife fights under moonlit skies, investigating cult activity and nefarious planetary plots - this is an *A*RPG. 

It's all about skills, loots, and explosions, and more power Neocore for doing that. 

1. The crusader is not a Space Marine. He is an ordinary human with extraordinary resources available to him. 

2. He is an inquisitor so he may requisition anything available to him. The only inquisitors capable of acquiring Terminator armour and Nemesis weapons are those from the Ordos Malleus, probably because of the (relatively) easy access to that technology due to the close relationship with their own chamber millitant, The Grey Knights.

3. Crusaders in a warband are usually protecting an inquisitor or tasked to protect something the inquisitor deemed worthy. As such, they are also given rare equipment not found in elsewhere in the Imperium. Powerswords are a rarity amongst Space Marine chapters, only sergeants and above have the authority to use then, yet they give them out to humans working alongside an inquisitor. Storm Shields are just as rare, even the Grey Knights can't get hold of them (joke) but they are a treasured relic among all chapters. A normal crusader only wears flakk armour, but he is no ordinary Crusader (see below) As an inquisitor, he is also capable of requisitioning power armour. While power armour is the iconic armour of Astartes, it can be created for humans but lacks certain features. It doesn't boost strength or toughness and it lacks all the sensors that the black carapace bestows. It is effectively just used for better protection. Because it lacks these additional benefits it doesn't require a fusion reactor in a back pack to operate. 

4. The Grey Knights are not the army of the inquisition. The inquisition has many different branches or Ordos. The three Majoris Ordos (major orders) are Xeno's, Malleus and Hereticus. Each Ordo has at their disposal a chamber militant. 

• The Ordo Hereticus (Witch Hunters) specialises in fighting the traitor (The enemy within). Their chamber militant are the Adepta Sororitas or the Sisters of Battle.
• The Ordo Xenos (Alien Hunters) specialises in fighting xenos or aliens (The enemy without). Their chamber militant is called the Deathwatch and has a selection of battle-brothers from the various Space Marine chapters.
• The Ordo Malleus (Daemon Hunters) specialise in fighting Daemons (The enemy beyond). Their chamber militant is the Grey Knights.

There are many more minoris ordos (minor orders) which do not have chamber militants. Each inquisitor, even from minor orders, is able to requisition anything they so chose. Adding Grey Knights as a sub class would not work because they are Astartes, inqusitors are not. It would be akin to adding Dark Eldar as a subclass. If expanded it might be possible for a whole new class stemming from the three chamber militants of the inqusition, each having their own sub classes. This game is called Inquisitor so I suspect Astartes (Space Marines) are not going to be playable. he best you could hope for is Terminator Armour and Nemesis Weapons.

What most people don't realise is that the various classes in the game are not those found in the inquisitorial warband. They are inquisitors. The classes only denote the background of the inquisitor before he was promoted and thus have a preferred weapon selection. As inquisitors they are armed with the Rosette and as such, speak with the authority of the Emperor Himself. Able to requisition anything he so chooses, from laspistols to Imperial Knights, at least in theory. His influence and that of the asset requested also play a part.

A good resource for all things 40K is called Lexicanum.