The hardware shouldn't be very complicated at all
- I've been thinking
about building up an 8031
(because I have lots, and development tools etc.).
If you use an AVR, you can use BASCOM from
www.mcselec.com which has
nice tools for that. You can build an AVR programmer for less than $5 and
you can have keyboard/mouse inputs and multiple serial outputs
But I'd use a pair of Attiny 13 or Attiny 2313 for this application...