If the cars only have ABS, the ABS control units are the same 996 to boxster. With PSM the control units are different. In addition to the control unit for ABS/PSM you need the wiring, the rate of turn/transverse accel sensor, the booster pump, the steering angle sensor and the PSM off switch and reprogram the instrument cluster. As you have stated, any upgrade to 5.7ABS needs the new wheel sensors which themselves have different connectors. Also if your 2000 has no traction control/PSM it is a 3 channel system and will need a new rear brake line and distribution block to convert it to a 4 channel system. Not something I would undertake frankly. If you can't drive a car without PSM then you should probably not be increasing its displacement
Todd