it can be done, but if you decide to do the entire car its a lot of work. i did mine off the car as i had the engine out and that was hard enough. just be prepared.
it really does tighten the whole car up, and is worth it in the long run. I would say the rear beam bushes were the hardest to do. i had to bore out the hole in the rear beam for the bush a lot so it would fit. even then it was a very tight fit!
if you are going to do any in the front, i would do them at the same time. arb bushes and wishbone bushes. take a look on the powerflex site for a diagram of there products on a mk3.