IH opponents don't generally need magic items. Being able to full attack everyone in reach, and then attack them all as they begin (or end) their turn next to them, is quite sufficient*. Champions, Demon Knights, Demonic Brutes and Dread Sorcerers are pretty nasty without items; really fun too. Using the IH PC classes also works well for certain types of foes.
What items they would have will generally be movement related (helm of teleportation, cloak of bat, boots of spider climb, etc).
Personally, I'd figure out what abilities were required to keep the fight interesting, granting those as 'permanent magical effects' upon the foe, and call it a day. But I'm rather stingy with my treasures.
If you want to go with magical gear, load up on potions and wondrous items. Don't worry about magic weapons and armor, nor about ability enhancers. Go for the fun items that give options rather than big numbers. Especially if you keep the IH defense / DR set up. Even if you don't (and that's half the fun), stick with the same cash but in consumables that fit the foes role and reason for being there.
*Whirlwind Attack 10 and Vorpal Hurricane 9