The big question is if you intend to use Windows (directly or emulated). If so, then an Intel based Mini is a must (VPC is a dog on a PPC). If not then any will do.
I use a CoreSolo, but there are still a few G4 users too. All of them are more than capable of running what at least 80% of carputer users need.
As far as drive size, that depends on how much media you plan to put on it. My 60GB is more than enough for me as I only have ~30GB of media. If you have more than 80GB (that you want the Mini to access), then you need more space (either an internal upgrade or additional external drives). If not, then 80 should be fine.