Rust is the worst of problems for any CIS fuel system. Unlike EFI the CIS runs fuel through its brains so contamination is a real bummer.
It's really bad for 107 cars as there has been a doubling of fuel tank prices in the last year. SLs are the only car that we regularly see tank problems (they have an internal plastic chamber in the bottom that causes its own problems).
If time is available there is a process done by franchises up north that can make good tanks out of bad ones. We have done a number of old Porsche 356 and 911 fuel tanks from them. The process is between two and three hundred dollars and the cut out perforated rust, sand blast the rest, coat the inside with a hard plastic liner and coat the outside with a pliable rubberized coating. We even did our Hyster forklift tank when they wanted $1200 for a new one. We use Moyer's Fuel Tank Renu in Pa (don't have more info at home).
if you think all this is expensive you should do it on a 1976 911 Turbo. I think the distributer alone was between 2k and 3k. One of the drawbacks to a CIS system and good reason for fuel filter service.
Bosch Master, ASE Master, L1
33 years MB technician