your going to want to trade the passat harness for a mk3 harness and here is why...
the mk3 harness incorporates everything into one bundle. the engine harness and chassis harness forward of the firewall are all one.
In the passat, the engine harness is its own standalone unit. so passat harnesses are highly desireable for people doing swaps in other passats, and mk1's and 2's
It will be very easy to sell and or trade the passat harness for a mk3 one. if you use a mk3 one, you unplug your gas harness, and plug in the diesel harnes.. at the fuse box... then plug all the plugs to the engine, and done.
if you use the passat harness, you will have to cut/remove all the protective coverings, from your golf wire harness, and systematically remove all the old wires that went to the gas computer/engine. its a PAIN IN THE ASS, as im doing it right now (except im removing everything but the engine harness because i have a mk3 harness for a mk2 tdi swap)
So. if you wanna sell that harness, hit me up, i'll buy it off you, and i know a guy that has a few mk3 harnesses.
ok so we got the wiring out of the way.... your going to need the downpipe obviously, and once thats out of the passat, and out of the way, you need the shifter box... it drops out from below, i believe one nut from above once you get all the covers off. (assuming your golf is an 8v and has the mk2 style shift linkage) then your going to need the instrument cluster (i dont see why your stock instrument harness wouldnt fit the tdi one) and the hydraulic setup for the clutch (you dont even need a reservoir, just run the lines together or plug the line that runs to the res) it should be an easy swap from there...