Yeah, definitely wait to buy an iMac. They’re announcing new models next week, probably including a 21.5-inch retina model.
Truth be told, though, I’m running a mid-2011 21.5-inch quad-core i7 iMac, 12GB of RAM, and it’s holding up pretty well. I’m running some 96k sessions right now (not my choice!), and certain plugins are hogs at 96k, but if you’re recording at 48k like a regular person, even “last year’s model” will be more than enough (although if you run a lot of samples, you’ll need a ton of RAM, and it’s not user-serviceable anymore — this is a HUGE problem). The i7 iMacs are beasts.
But wait for the new ones, even if you’re going to buy an old one.