So does the Grey Knights book have to have Grey Knights (that sounds like a silly question now that I wrote it). And what is a Grey, Knight just a space marine or something more? If they are just a space marines with a fancy name it could work.
You don't have to include grey knights, though you do have to use the special character Inquisitor Coteaz so that you can have inquisitorial henchmen as troops. Actually, your old terminator would do to represent him - he has artificer armor and a force hammer.
A second inquisitor can be armed in various ways, including with some alien items. Also you can have assassins (vindicare, eversor, calidus, culexus).
Grey knights are just space marines with stormbolters and force weapons. Also, inquisitorial warrior henchmen can be armed as space marines.
Here's an overview on henchmen:
3-12 per unit, any combination of these:
-arcoflagellant (close combat cyborg thing)
-banisher (chainsaw priest)
-crusader (power sword and storm shield)
-demonhost (could represent an odd alien creature)
- deathcult assassin (fast killer with two powerswords)
-servitor (can have heavy weapons)
-Jokaero (techno-monkey)
-mystic (guides in deepstrikers)
-psyker (psychic artillery)
-warrior (like a Ld 8 guardsman. Has laspistol/chainsword and flak armor. Can have bolter, stormbolter, or hot shot lasgun. Can also have carapace or power armor. And meltabombs. 3 per squad can have melta/plasma/flamers, combi-melta/plasma/flamer, power sword, plasma pistol, power fist, stormshield.)
Vehicle-wise you have rhinos, razorbacks, chimeras, land raiders (three types), stormraven gunships, dreadnoughts (venerable and normal), and the dreadknight (guy in a big robot suit thing).