The only significant discounts you can get on Apple hardware are student discounts. If you have a friend or family still in school (or a teacher), it might be worth it to look.

The iMac can take an external monitor, FWIW. Up to 1920x1200 for external digital monitors. I don't know what resolution your 30"er is, but probably more than that.

MacOS has hierarchical folders just like Windows does. It just doesn't have the "drive letter" concept. I can't tell you for sure about iTunes (I don't use it on either platform) but I'd be amazed if the Windows version had a capability that the MacOS version didn't. (I did use iTunes on a Mac for a while ages ago, and you could definitely keep your mp3s in their original location and not have to put them all inside iTunes' datastore.)

Why not consider the Mac Mini? It's not as fast, but it's certainly fast enough, and it's far cheaper. Oh, it only supports 1920x1200, too.