Also, would he be risking his neck if I put a Necromancer's robe on him and had him walk around in public?
The only necromancer's robe you can get is Mannimarco's, or you can add one with the console, Construction Set, or probably a mod. Members of the Mages Guild won't interact with you except to gripe about the robe, if you add a regular one; they won't care about you wearing Mannimarco's. Far as I know, nobody else will care about either one, including other necromancers (they'll still be hostile). Once you get Conjuration to 70 though, people will sometimes comment on how you "smell of death."
However, vampires sometimes have black robes, and I've found black hoods somewhere as well although I don't remember where (wasn't from killing named NPC's). The DB robes would probably work just as well. Might be worthwhile to join the Mages Guild and do the questline, at least as far as getting the Necromancer's Amulet -- you could always be skirting that dark side, then when you can't resist the temptation to wear the Amulet, you decide to keep it and go be a necromancer :shrug:
For decor, if you have Shivering Isles you'll probably want to go find one of the shambles skulls, and go to Crucible to do Ushnar gro-Shadborgob's quest for the skinned hound pet -- since the hound is easily killed, it might be better as a container in your house.