I think AGP or Quicksilver G4s are nice for that use case. They have lots of ports and stuff for expansion, their RAM is easier to get than pre-G3 EDO DIMMs, they can have decent GPUs. I would avoid MDDs, they run very hot and have very noisy fans.

I have a soft spot for the 9600, which can take PCI cards for things like USB or SATA, but their RAM modules are hard to get and their CPUs are very slow. I would also avoid anything older than that, except if you really like the hardware. It’s very difficult to find NuBus expansion cards.