I'd go for a Mac with MorphOS. The mini is a really cute and powerful maschine. But if you want to keep the expense at a minimum the Power Macs seems a smart choice. There's also the option for very cheap eMacs G4 (their disadvanate of course is the inbuild CRT).
The 1.25 GHz Power Macs are good to get for around $/EUR100. The minis are a bit more expensive, but I like the small form factor very much. MorphOS on the mini works pretty well and having OS X seems to be not a disadvantage to me, too (but haven't booted into OS X for weeks now).
