The one I've been looking at are the Omega, the BMW 3 series (E36) and the Mercedes 190.

All of them are significantly more difficult to use than the Sierra bits, Some bits are easy to accomodate (Diffs for example) but the multi link rear suspensions mean that it is difficult to incorporate the bearings/hub into a new wishbone system. It may be possible to cut the bearing carrier out and weld it to a similar upright to the Roadster or have a bearing carrier turned up on the lathe.

The fronts have different problems, the BMW has a very short upright which means that a new fabrication is required, the Omega has rearward facing steering arms. This means that the steering rack would be in the same place as the sump/crankcase in a Roadster. This might be solveable by swapping the uprights side-for-side.

I've not had a detailed look at the Merc bits but I believe that that too is a "rear steerer".

I don't know wether the RX8 is RWD or not.

Other possible rear axle donors are four-wheel-drive versions of front drive hatches or saloons. The Golf 4motion and the Peugeot 405 come to mind. There is a very rare 4x4 Mondeo too.

If we can get the parts It should be possible to use them but it won't be easy!

Cheers

Chris