The gas tanks need urgent attention.
The seller of the car already told me the left hand side tank was leaking. I've poured some (old) gasoline in it and here's the result after a couple of minutes:
Some of the horrible sludge that came out of the "good" tank when I took the sender out and there was more petrol in it than I had thought...
There are two possible approaches: buy new tanks (in which case I'll probably have to buy new senders and maybe even new fuel pumps to fit the only replacement that still exists) or try to repair these. Matthys sells kits to derust, degrease and coat tanks, and even little holes should be no problem. I'll keep you updated.
I went for the largest kit (110-170 liters) because I figure I can do both tanks *and* the Mk II tank with this kit.
Both senders are defective. I've only taken out one of them (the one from the good tank) and one of the conductors (metal strips) had sheared off. They can be soldered. This isn't my best soldering ever but it was only a trial. I have to take out the sender again when I coat the tanks.