[trekiej]

You have to worry about keyboard and mouse compatibility if you buy one of the older systems. IIRC, the Personal Iris and Iris Indigo use proprietary keyboards and mice.

Correct, the older systems (Professional IRIS, PowerSeries, Crimson, Onyx, Personal IRIS and Indigo) used serial keyboards with DB9, DB15 or mini-DIN connectors. That last one is tricky because it *looks* like a PS/2 connector but it isn't. Magic smoke will escape if you attach a PS/2 keyboard to an Indigo or Onyx. Their keyboards can be recognized because the cable is separate, and the mouse attaches to the keyboard with a short ( ~ 50cm) cable rather than with the standard 6ft cable directly to the system itself. When in doubt google the part numbers on the keyboard/mouse before using them.

The pre-PS/2 keyboards and mice are interchangeable: you can use the keyboard and mouse of an Indigo (mini-DIN) with a PowerSeries (DB15) or vice versa if you have the right adapter cable. I do this all the time because I dislike the older optical mice (the ones that need the reflective metal mouse pad with the grid)

Ugh, sounds tricky.
Edit:
I will be happy to stop by here to get help before I start.