Stainless is a perfectly good material for tanks. A good welder should be able to look at the tank and tell you whether it is worth repairing. If you have an air source, you could pressurize the tank on your own and put a soapy liquid on the seams to see pinhole leaks. If there are lots of pinholes, it will be better to replace the tank but if there is only one problem area, it can certainly be ground and rewelded.
My own experience has been that stainless tanks go something like 40 years before needing major work. Tanks under pressure tend to develop pinhole leaks in welds earlier.