I own a 77 step side. The replacement fenders can be found through several vendors, LMC, Dennis-Carpenter, and I think the Motorhaven. Bottom line, they are readily availble and not too expensive. The rest of the box parts are availble trough the same sources also. I received a catalog in the mail last week from macsutoparts, and they even have the bed wood kits.
The fuel tank on mine is a I believe a 15 gal, it is rear mounted. The ranger XLT is just a trim kit, which means you get more chrome goodies. me personally, I like the looks of the stepside with out all the chrome.
If the truck itself is pretty solid, I would get it. It's not everyday you see early model stepside. They are very good looking trucks.
Check out my galley and you can get a good idea as to what these look like. Mine is a custom, so it didn't have all the body chrome.
hope this helps ya!