Well some stuff you'll need off the top of my head:
Complete GTI front brakes inc calipers and possibly hubs?
Complete GTI engine including ECU and bay wiring loom
Both GTI fuel pumps, under car and in tank inc loom
Complete GTI rear beam, handbrake cables & brake pipes if you want rear discs, the beam will include an ARB + pressure compensating valve
GTI ARB & fittings, you might want to go for an uprated one not essential though
Complete GTI suspension kit
Nothing too serious all bolt on/bolt off jobs

Engine wise I'd think about a 2.0 8v or 16v from a MK3, should give you plenty of power for a stripped down MK2

Only problem would be wiring it in, as the 1.3 will have lots of missing loom plus it is pre-CE2 so nothing will plug straight in. If it was me I'd try to get a mostly complete MK3 and strip the engine and as much loom as possible including the fusebox. Then I'd get a late MK2 dash cluster loom and dials, along with a late MK2 ignition housing & stalks. Then it should all just plug in nicely no fuss
