How should I go about undercoating this truck? (pics)

[chevyman95]

Got an 83 Scottsdale from my brother in law, not too bad just needs a lot of TLC. Great 383 stroker in it. I want to put it back on the road maybe lower it back down but it wont be used as a mud truck. I am going to paint the exterior but want to get the underside looking good first. I was planning on pressure washing the dirt and knocking off any rust and then either spraying it with rustoleum or por15. Don't really want to spend more than about $200 doing it but I would like for it to look decent and last a while. I am hoping to take off the bed but leave the cab on the truck during this process. Whatever I put on it needs to be able to go on rust because I am not sandblasting it because I don't have the equipment. Any writeups on doing this? thoughts? tips? opinions?