After further thinking; A 3620 is suficcient. The UI is so responsive that it'll work at 4mhz. What one needs to reverse engineer on the 3620 is the HDD, and the fragile console.
The biggest challenge is the keyoard.
The easiest approach is probably to desolder the hall effect switches, take a datahand and solder them together.
This is however inefficient since with a chording keyboard one can have one hand on the board, and one hand on the mouse, and still get full features.
This is troublesome however, the assembly is not that hard, but getting the key combinatioins to work software wise can be troublesome(mostly because of beastie's "Okeg is not allowed to write drivers" ban heh)