The question is, can it be done?
I have a pretty decent collection of Space Marines painted up as Exorcists, a daemon-hunting chapter that is probably successors of the Grey Knights. According to the fluff, they are basically organized and fight just like a normal Space Marine chapter, their heritage manifesting in an unusual resistance of daemonic corruption and possession and a tendency to fight with measured discipline, rather than righteous fury. However, it might be fun to cook up a list using the Grey Knights codex for once in a while. I'd even be willing to make a couple of conversions - hacking together a non-Grey Knights-style Exorcists holding a psycannon or incinerator or whatever janked out Grey Knight weapon I want to use - though I would be against buying entire unit boxes. If I wanted to do that, I'd just play Grey Knights.
Which I might do anyway. Because, you know, you can never have too many armies*
As I wrote above, the question is, can it be done? Assuming that my army is mostly made of ordinary Space Marine bolter-brothers, can I field them as Grey Knights without causing too many WYSIWYG headaches?
The parameters I am setting myself are this:
1) I am willing to stretch WYSIWYG in straightforward, squad-wide ways. For example "these boltguns are all going to act like storm bolters, which is easy to remember because there are no ordinary bolters in this army" or "all these guys have the same melee weapon, not included in the model."
2) I am willing to use eBay to buy a few extra bits, so for example, if my 10-man tactical squad includes a plasma cannon and a plasma gun, I'm willing to buy the bits it takes to turn them into a pair of - say - incinerators.
3) I would be willing to buy for my Exorcists stuff that works well in their Grey Knights version, but not things that I can't use/would suck for my Exorcists. If I was going to do that... I'd just play Grey Knights.
4) I like mech armies (and I cannot lie...).
This is, in part, a decision-making thread, so if you want to cut to the chase and say "don't bother - it's a dumb idea" or "do it, it's totally doable" or "don't play Grey Knights, they're boring" or "just buy some Grey Knights, you wimp," I'm willing to read that, too. However, I am also interested in exploring stretching WYSIWYG - both in general and in this case - and I'd be happy if this thread included some discussion as well.
* This is a lie; you can totally have too many armies.