I guess we would have to know what you plan on doing with the truck before we could suggest mods. I guess my first buys would be some recovery gear. A winch is out of the $500 budget, but you could pick up a Hi-Lift Jack, good tow rope, a heavy duty come-a-long (not the cheapy sheet metal ones you get at the hardware store) and a folding Army shovel. This stuff should get you out of a stuck, with or without another truck around (although you should never go wheelin' alone). Also, invest in a good first aid kit that has more than just some neosporin and a half dozen band aids. If youv'e got some left over after that, a quality set of off road lights would come in handy as well! Good luck.
1983 CJ-7 Laredo