The sky is the limit.
Being practical about it the best way to approach though.
I don't think Orks would care much about a purity seal. It just doesn't seem like something they would bother to loot - but hey, its your army and whatever you want to add is fine.
Guns would be the most ideal looting item, as it seems fair to say that the average Ork would admire Imperial dakka.
I've been painting a looter squad of Grots, and one is wearing an Guardsmen helmet, one has a Guardsman canteen and the rest are sporting various
IG and
SM weaponry.
Get creative, try to imagine what appeals to an Ork.
I made a post a few days ago in the "So what are you doing today?" thread with this guy, but I suppose this is relevant enough to post here again.
A lot more paint has been splashed on this miniature since this photo was taken, but I'm waiting on a few more paints to arrive to finish him before posting completed pics.