Money To Burn
Spending imaginary money is always fun. If you are an arcane caster that is up there in levels SCR*W AC. You will not get enough to really protect you. The cloak of displacement might be about your best option here. But mainly depend on your spells and shrewd maneuvering in combat for that. What you want to be able to do is shrug off debilitating spells (resistance, spell turning, and SR stuff).
But the most important thing is to be able to bring the mostest the fastest. In my opinion you want stuff that will jack up the speed (how many spells), Lethality (damage), and DC (how many will stick), of the spells you throw. I would certainly look at any items that would let you do this like the metamagic staves etc. High level rings of wizardry also. Hit em fast, hard, and make em count. Get a staff that lets you quicken a spell and get something that lets you Maximize a spell. From the way your group plays it seems like there is a big fight or maybe two then everyone goes to rest again so unloading as much smack as possibly seems pretty important to me.