I can't speak precisely to the interchangeability, but if you plan to go the wrecking yard route, most yards will tell you what years are cross-compatible. If buying new/aftermarket, it seems like the suppliers usually are pretty accurate on interchangeability.
Sounds like you may reside in the rust belt. As a former resident of the rust belt, historically I did several things to try to stave-off the tin worm, but ultimately it will likely win the war. You may just prolong the inevitable to some extent.
Prevention is a big help. While my vehicles were in use in winter, I regularly/frequently/obsessively washed the road salt off as soon as I could and that didn't mean waiting until spring. Road salt (mostly sodium chloride with some other additives) is water soluble and the less time it spends on vehicle components, the better. The part most people neglect is washing the vehicle undersides, undercarriage, suspension/brake components, radiator/condenser, etc. - not just the visible cosmetic surfaces. Any place salt might have seeped in, I flooded with the garden hose to try flush it out. It was a war of dilution, the more water, the better. Drive through car washes are basically worthless. Along with the above, it is very important to remove leaves, twigs, road debris in any places/crevices where it can accumulate on the car (e.g., windshield plenums, inside wheel wells, under fender lips, dead spots, you get the idea). Such stuff can act like a sponge and can speed the onset of corrosion by constantly/prolonging the areas stays salty/wet.
Moving on to replacement body panels. Whenever I sourced a good panel like a fender, first thing was to wash the inside areas/cracks/crevices thoroughly to help adhesion of the protectant(s) I would be applying. If there was ANY pre-existing surface rust, I would treat the spots with a latex-based rust conversion product (there are multiples out there) usually looks milky white in the stock container, brushes on and turns the rust to black and then dries - you've probably seen/used it. I have, depending upon the part in question and how much time I had to mess around with the repair/replacement, I would use either a decent rubberized, spray-on undercoating sourced from the auto parts store, or, Rust-o-leum oil-based, brush on paint, usually bought in quart cans. Whatever product I used, it was always applied in multiple light coats allowing for complete drying after each coat. I always made certain to get the product into every crack and crevice. I usually applied to cover as much of the inside areas of the fender or other sheetmetal part such that I thought it would be sufficient. That usually provided decent protection for the INSIDE of the part once installed as I would likely not see that area again unless I needed to get in there for some reason. Note: trying to use rust preventatives like the above while the part is installed won't ever be as good as doing it on the part when it is sitting on the workbench off the car as you won't have comparable access.
On the outside of the part, the biggest thing was to try to keep up with any chips or scratches in the paint which would allow for salt to get to the bare metal under the paint. Once salt got in and began the corrosion process, it usually turned into a bigger job thereafter. If corrosion started, I'd treat the spot with the aforementioned latex-based rust converter, allow to dry thoroughly and then use touch-up paint if needed.
Sorry, probably longer than you wanted, but I hope it helps.
