I you really just want to keep and old car from dying of rust but can't (ever) spend a bunch of money on it, just coat the affected surfaces with a good corrosion inhibitor.
Clean it well, treat it with some kind of metal prep like ospho, then paint or spray some serious gunk on there like fluid film, lps3, rustlick, or even better CRC SP-400.
I ended up buying the 5-gal of crc from Zoro (with a 20%off code & free shipped). They don't make sp400 in a 1 gallon (wtf) and the spray can version is terrible - super toxic, have to wear a respirator, doesn't last at all. The liquid version is super easy to work with a goes a long way.
I have way more than I wanted but hey, it's forever we're talking about
https://www.zoro.com/crc-corrosion-inhibitor-5-gal-03286/i/G1497246/
Zoro is friggin awesome