I wouldn't suggest a G4 mini, unless you want a server with no expandability. Even if you're fine with being stuck on Leopard you're limited to a maximum of 1GB of RAM. If you're looking at a G4, getting a PowerMac G4 is the way to go.
If you can, have a look at Core Solo minis, they're pretty cheap, and for ~£40 you can drop in a Core2Duo and upgrade to Lion (other wise Core Solo/Duo are stuck on Snow Leopard).