Still running a 350Mhz B&W G3 as my router/gateway and DNS, mail, and web server (10.2 Jaguar Server).
It's been running 24/7 for going on 8+ years (except for the odd power outage out lasting the UPS).
One of these days it'll get replaced by a 933Mhz G4 with 10.5 Server. Mostly because of improvements in mail service.
I wouldn't worry too much about the Mini itself. The HDD or optical might need work at some point, but even then you might be surprised (the B&W's HDDs have been running for most of the last 8+ years).
Although I'm running some vintage h/w for the servers (and have a moderate collection of classic and PPC models), we do have fairly late model Intel portables, iPads, and iPhones for our individual needs. I just don't need a lot of horsepower for the server.