If the rear fenders are affected, that is rather bad, as welding has to be done for a proper repair. It's time-consuming. If there is rust on the front fenders and the tailgate, that's no problem. You can get used spare parts, already painted in the right color, on ebay. The installation is done in 2 hours for a fender.
If your specimen is rust-free - spray the inside of the wheel arches with cavity spray or preservation wax. The cause of the rust is rotten sealant, where water then enters the seam. It is therefore advisable to re-seal if necessary, Rust should then no longer be an issue.
Otherwise, the sheet metal of the E36 is very resistant. Even with rust spots, these can stay the same for years without causing a hole.