This is an old post but I will toss in my 2 cents and give a little input from auto interior work...
Sound proofing or more correctly "shutting the damn pig up" is a lot less complicated than it sounds. Standard carpeting pile is what 99% of auto shops use its cheap and is sold at hardware stores everywhere, this is applied to the sheet metal with spray on contact cement and then the carpet or liner installed over it... Cutting, shaping, layering etc are as simple as a razor blade and a little time.
I would suggest undercoating and a few days of drying before doing anything. Undercoating is rough and textured which helps break up the sound waves, how its applied is up to you (roll or spray) just keep the thickness under 1/8 of an inch. To thick and it cracks and peels easily.
To test out the difference here is a quick and simple route... Go buy a piece of standard household carpet pile (about 20$ worth will do it) cut a few pieces and toss them into the floor of your vehicle and take a ride... You will notice a HUGE difference...
This is also a great way to hide wiring, A/C and heat ducts, etc... The wiring or rectangular ducting are laid out in the vehicle floor, a layer of pile is installed over them, the pile is trimmed to the routing and a second layer is installed over it. This helps float out and hide the humps and bumps.
To make it all removable carefully lay out the pile overlapping and gluing the edges of each piece to each other... Spray the final layer with contact cement and glue carpeting over it... Then you can pull the pile/carpet out in one piece to dry or do work to the inside.
Doors can be lined with the same materials, cabs, walls, etc can be lined the same or made as removable panels by using 3/16 ply (door skin material) or even insulation panel, cut to shape, covered in foam or pile, then carpet, vinyl, cloth, etc... It sounds to easy to be true but this is exactly how $10,000 custom interiors are done...
