I've built four booths, the first was a "not very good one". The last three were cheap and great. A spraqy booth should be 24 by 14 feet. The height should be 10 feet if possible. I framed them with metal studs--2x4--on 16" centers. at the door openings i doubled the studs, one inside of another, I used 2x6 metal studs on the ceiling on 12" centers--it doesn't cost that much more for the strength gained. I used wind braces for diagonal support. I lined it with white barn metal, not galvinized because of the weight. Run the metal horizontally and caulk the heck out of it as you install it. For the ventilation I had the fans  with air ducts and filerts about 6 feet from the booth and ducted the air into the ceiling corners and the center corners of the ceiling. The exhaust was through two ducts under the area where a car would sit just laying on the floor. The were heat ducts made to go inside of walls of a house, an oval or rectangular shape, they make both. these ducts were parallel and running lengthdwise nearly the full length of the car. They are made to go in a 3 1/2" wall, so they clear most cars. If they wont clear, lay 2x6 boards on the floor to drive in on and raise the car 1 1/2". cut about 3" holes in the side of the ducts for the air to go through' four per outer side of duct. Install duct work on the ends of the booth for these to slide on to. Don't make this a perminent attachment, leave it so that the ducts can be slid off of the ducting at the wall when not in use. Now you have a pressurized booth that wont draw dust and oil in when in use. Place your lights verticly on the walls so that they extend at least a foot above any car you are painting. put he ceiling lights cross ways in the booth . Put the end lights about 2 feet from the end of the booth. You can add more lights, the more the better.
A good and cheap place to get lights and studs, and even a walk in door is Your local Home For Humanity outlet. Construction companys donate materials and you can get it for cents on the dollar, and the best part is that it is a tax deductable contribution. Hope this helps someone!