Seafoam works wonders on a dirty engine, but it sounds like you might end up going through another filter or two before your fuel system is in good shape.Try going back down to Upap and grab those fuel lines off of the mk1 down there. It'll make life a bit easier if you can clean and install those lines at the same time you put your new tank in. Probably your only option for new lines is to make them yourself. If the mk1 is anything like a mk2, then the fuel pump is easy to change since it is in a plastic box mounted under the car. One banjo fitting, two wires and three screws and it's out, but it will dump fuel all over your chest in the process if you lay under it while you do it. If you want new injectors buy some for a 1986 mercedes 190E. The only difference between those and your current injectors is that these are brass and a 1/3 of the price (~$30). If you need used injectors you can get them off of any CIS equipped car (Volvo, Mercedes, Audi, VW, Ferrari, Saab, etc.) except for the 2.0l 16v CIS-Motronic cars (they have a different thread pitch and won't screw into your fuel lines but you can buy adapters for them).
Start with fixing things you know need to be fixed. Whether it's the problem or not, it needs to be done and it's something you can cross off of your list. Try pulling your injectors and do a spray test. If you haven't already changed out your injector seals with new ones you'll have to do it when you yank your injectors out. See if it's a nice cone, a dribble, or somewhere inbetween. More than likely they won't spray a perfect cone, but as long as it is a good spray and does not look more like a stream it should be ok to use. It sounds like you might have gotten some junk in your fuel distributor. You can pull the piston out of the bottom of the FD and clean it, but you can't disassemble the rest of the FD without destroying it. Might be a good idea to pick up a junkyard spare since rebuilt they're over $600. What type of fuel filter did you put on? I've seen with some of the cheap ones (Fram) that if you don't fill it first with gas before you install it the pressure from the pump will blow the filter element off of its fitting inside the filter casing and allow unfiltered fuel to go past into the sensitive parts. I learned this lesson on my sister's jetta. Now I only pick up fuelfilters from Bap or online retailers that specialize in VAG products.
