Look for something that has all of the basics there. A shell just means frame, roof, rear quarter panels usually. You will save a lot more money if you start with something that actually runs at the time you buy it. It doesn't necessarily have to be completely roadworthy, just in decent mechanical order. Trust me, there will still be plenty of work to do!! Also, unless you want to do a lot of body work/ welding look for one with little rust. Check floor pans and front/rear frame rails very carefully. I had to replace almost all the floor and rear frame rails on mine (it was a fun and education project, but you need equipment and money for those parts). Good luck finding your project car!
-Tom