Cosmetic rust like door bottoms, rocker panels and truck beds around the wheel wells can be fixed for cheap by replacing pieces with color matched parts from a junkyard that aren't already rotted out. Cab corners and such aren't so easy to replace, usually they need re-formed with filler or new steel, which isn't cheap.

At 18 years old, frame rust is what you should be more concerned about, not cosmetic rust. It'll never be a collector's item. It's 18 years old, every truck in the rust belt gets rusty by that age. If it's ever gone off road, kicking up rocks in the rear wheel wells chips the paint and starts the rust there. Likewise with the outer parts of the doors and the rocker panels. They also rust from inside if you don't keep the weep holes clear. Most people don't think about those. My girlfriend's dad has a '99 Dodge Ram 2500, the body is pretty rusted out, but the frame's solid. 

I would avoid the expanding foam
Better to form some sheet metal and epoxy in place 
smooth it out with body filler and paint
As far as the bed, Try to locate 1 at a junk yard etc
Whatever you do remove ALL the rust before body repair

fighting rust after it's started is a waste of energy in my opinion.

Just hit it with some spray paint once in a while so it doesn't look as bad and use your time for something more productive.

Agree with Justin. Accept the cosmetic rust and be more concerned about the frame, brake lines and suspension rust.