With regards to MorphOS, the MPC8610 appears to be the most attractive choice out of the three that were mentioned. I think the key advantage of any new hardware would be the option of using graphics cards that include more than 64MB of memory. Irregardless of the CPU performance and potential warranty issues, this would certainly justify the added cost compared to second hand Macs.
The Mac Mini is a great machine in many ways, but the graphics memory is a bit limited when you are dealing with high resolutions and many (virtual) screens. A number of Pegasos owners already use cards with 128MB of graphics memory for this reason.