I've built lumber racks with lightweight angles, then later made a plywood piece on each side cut to fit into the angles frame. The sideboards were easy on/easy off so I could put em on when needed or take em down when not needed. It only takes about 4 screws to hold each side in place. Depending on the truck bed sides, the base angles can be bolted in only a couple of places without fear of safety. If you're concerned about the paint just lay a strip of rubber under the angle before bolting it down. With the top angles just above the cab height you can carry some fairly long stuff. Only thing is the back cross member can be in the way for loading taller items into the bed, so I adapted it to be an easy on/off too.
Skill level is low to medium, and even at todays prices, if you had to buy new materials(I think I used bed rails, which are extremely handy hobby material, for the base angles on mine) I think under $50 would build a nice lumber/ladder/sideboard rack