Bare minimum needed for rear disc and 5x114.3 conversion:
Front P5/ES wheel bearings and hub (not the spindle, the hub and bearing can be pressed into 4lug hub), front P5/ES rotors, P5/ES rear hub and spindle, P5/ES e-brake cable, P5/ES rear rotors and calipers.
P5 and ES parts are your best bet, easy to find and cheap.