I would strip the suspected areas with a Norton paint stripper disc on a grinder to see if the fender is rusted out or just dented up. Then if you feel that you are up to the task of fixing the dents, have at it. If it is banged up beyond your ability to fix it, find a better used one. I am in the same boat as you are. I fixed all the rusted areas and now I have to decide whether to bondo the many rippled areas on the rest of the fender or look for a better fender.
