Are you just trying to make it functional or do you want this to look good? Back in the good old days as a teen with little money and only old rusty cars to choose from we would use sheet metal and a pop rivet gun. As for rust inhibitors, I've only had limited success with the few I've tried. The best results have always come from just removing all the rust (including the deep down black spots). One of the few tools I could afford was a spot sandblaster and an old cobbed together air compressor.