software
The system should be a full-fledged x86 compatible computer, running off the shelf software.
I am NOT and will NEVER advocating any OS. It is obvious that for some applications, a Linux/Unix setup is probably the best solutions, whereas for other applications, a MS Windows system (or even another OS) would be a good choice either (e.g. for production computers).
Ideally, the storage device should be swappable (e.g. compact-card harddisk like the microdrives), in order to quickly swap disk images for the required setup.
All software considered for the project should however be tested and/or tweaked and optimized for low-power consumption. This will probably be achieved at the cost of performance, yet this seems acceptable.