For more parts leads, your 1984 Rabbit looks to be identical under the hood to my 1987 Scirocco. Same intake manifold and solid lifter head exactly. You could just order an alternator for a Scirocco with air conditioning -90A. Idle speed is controlled by that solenoid bolted to the strut tower and relay #1, which I have had to replace. Have you looked inside the fuel tank? Certainly the strainer inside must be replaced. My tank even had rust from the half full point up.
On your transmission can you wobble either drive flange? The driver side one is under heavy side loading. Bad motor mountings will do evil stuff to the axles and drive flanges. I have had to replace the output seals and drive flanges. Does either one have 100mm CV joints?
I too was wondering about checking the oil pump since mine has even more miles on it than your Rabbit. Adjusting the valves, after far too many miles, reduced oil burning.
1984 Audi 4000 1.8L is same setup as a 1989 Fox. The intake manifolds do look different than what have. Audi one is closer to a cylinder. Aux air regulator is on other end below throttle. CSV sticks out at odd angle , fine for a Fox, but on Rabbit or Scirocco is pointed at firewall. Here is a link about modifying the manifold to put the CSV at the far end: http://www.scirocco.org/tech/mk2/5000tb/5000tb.html
I just happen to have a manifold for a 1984 Audi 4000S 1.8L (049133223AA 1H WWO) sitting in my garage from a used head I bought. On the CSV end is a 29mm boss with a 17mm plugged hole in it. Interested in it? Have to ship from U.S.