Remove fuel tank and inspect the inside. Have the tank cleaned out or do it yourselves. Try to see what was causing the problem.

If tank will not clean for some reason or you see rust. Get another tank from an auto wrecker and make purchase depend on a clean usable interior condition.

A fuel tank is made of normal steel that is coated inside and out with a layer of lead basically at the time the metal is rolled. In north america it is called tern steel if my memory is right. Possibly the name of the factory the specialty steel originates at.

The interior coating that prevents rust can fail but it is very unusual if it does.. Generally the issue may not be rust but you do not know until you inspect the interior of the tank. For example someone could have intentionally put something in the tank or the rust could have been pumped in.

Try to think a few hundred dollars at worse if you do it yourself. It is pretty certain the injection pump is still good so $4800.00 is hard to understand.

I would for a host of reasons install a fuel pressure gauge in the system permanently as the lift pump might be suspect or the fuel pressure low. Cost about twenty dollars to do this yourselves. After the tank is checked and filters renewed.

If you insist on taking a car or your friend does for all paid for service on a car this old you should have very deep pockets. The sooner one takes on repairs themselves. Overall the more money is saved.

Most required tools pay for themselves or more the first time used. One of the few things in life that I have purchased that have absolutly paid their initial cost many many times over is tools.

The only thing I am certain of from your description is the tank has to be removed and inspected.