I could not find a definitive answer for this on this forum, so I'm asking here:

If I only wanted to use the Grey Knight rules from Dark Heresy, JUST the Grey Knights, do I need to own the core book, or does Daemon Hunter and the DW Core rulebook cover all necessary rules for Grey Knights Characters (whatever required adaptation for item cost, etc. notwithstanding)?

You should be good with just Daemonhunter. The rules are similar enough across both books. What you will lack, is context for how the GK psychic powers used to work in the DH psychic system, but it should be clear enough on how to use them in the DW system.

Ah, that's good to know. I was interested by the admittedly far-flung idea brought up in a thread involving the Grey Knights here about a Black Shield in Aegis Armor. There would have to be a VERY good explanation for why a Grey Knight would not choose to find his way back to his own chapter, but I could imagine a Watch Commander, if he even knew of the Grey Knights in any detail, seeing the advantage of accepting such a warrior. After all, they already have a special Auspex stolen from the Ordo Malleus.

I'm thinking such a character could bring whatever equipment they had on them, but have to go with the Deathwatch armory for other things for obvious reasons. Anyway, just an idea. Thanks!

I was mucking about with solo and squad mode abilities for GKs, I know the book says that the GKs use their psychic powers as their chapter solo?squad mode, but it just didn't make sense to me. I'm thinking about giving DW squads some cohesion penalties for having a GK in it (he has to spend an extra point to use powers or something).

Here is a very, very rough draft of the powers: Link