Hi,
From a purely selfish point of view, I'd go for a Z80-based CPU, but that's only because it's the CPU I'm most familiar with.
For my HND (UK exam qualification) project many moons ago, I pulled apart a multi-user CP/M machine that consisted of five lots of CPU/RAM/serial I/O boards (one for each user!) and a further board containing the interface logic for its shared 10Mb hard drive.
I took one of the CPU/RAM/RS232 boards, disassembled its 4Kb boot EPROM, doubled the ROM address space from 4Kb to 8Kb, replaced it with my own OS (fairly rudimentary!), added a PIO (parallel input/output) and finally added an AY-3-8912 sound chip. I made a board containing 16 LEDs and connected that to the PIO (so I could make "chasing LED" effects), and also had the AY-3-8912 playing ripped ZX Spectrum game music.
All in all, it was a great learning experience for real low-level hardware stuff. Of course, this was all easy to solder, non-surface-mount and the hardware I added was created on standard stripboard so it was cheap too.
Anyway.... if you're targetting the Amiga audience, I guess the 680x0/Coldfire would be a better place to start...
- Ali
