My understanding for the R56 is to buy a car with the sport suspension package and then swap in the smaller front sway bar from a non-Sport car... or vice-versa... the only difference is the sway bars on these cars.
Apparently, the R50 is a bit different. The sway bars and the springs were part of the factory SS+ package, so here the plan is to get a non-sport car and swap in the springs and larger RSB from a SS car.
However, having dropped the front and rear sub-frames a few times on MINI's (R53's mainly), I'd be hard pressed to say which one is more difficult. The front sub-frame is scarier if you haven't done it before... the rear is more annoying, for some reason...
Have fun!
(also running a Cooper in HS)