Yeah rear beam is a straight fit, but if you also upgrade to disc brakes you'll need to add a brake compensator and a couple of flexi hoses.
I believe the mk3 rear beams are all roughly the same width, search google as think there's a thread on clubgti where someone measured a load of beams.
To widetrack front you can go mk3 but will be 5x100. To stay 4x100 is more hassle then it's worth really, passat parts I believe, again search google and lots of threads.