My new motors arrived from Oatley Electronics on Friday... They're a bit heavier than they look - but hardly surprising when you think about it, really. Brushed motors, which means that they're not as efficient and won't last as long as would be ideal - but so much easier to drive than any other type, such as brushless.
They're 100W at 24V, so they'll put out a bit of power, while not needing too much amperage. Of course, it means playing around, and plenty of batteries in order to get the full power... On the other hand, because they are brushed motors, it will be possible to run them at lower voltages - and therefore lower power.
I've just checked from the standardised astromech plans (available online), and it looks like there will be plenty of room in the feet to fit them - the whole frame is about 5 inches wide, 3.5" at the top, while feet are 7 wide at the bottom but 3" or so at the top... What I will do is do up a paper foot to double check... If I have to fudge... I suppose it will have to be... I doubt any hobby builder would call me out about it - once you know how much work it is to do, you don't denigrate others... If someone does, I reckon that a couple of hundred watts propelling quite a few kilograms bearing down on them will change their minds.
The other step is to build a couple of motor driver boards - a few high power MOSFETs, a current shunt (to test for motor stall, etc., rotation encoders...