Class 11, it's not a sexy class, depending on what you want to do for a motor, I think one could be built for under $5k, if you're a jedi master of the classifieds you could do it for $4k. Class 9 wouldn't be much more. But, and it's a big but, neither a class 11 or 9 is going to be a real fun desert toy when you don't want to race, for that I think your best bet is a 1450/1400 truck, or maybe a 5-1600. The problem with 5-1600 is the majority of the field are serious racers who stretch the rules to the fullest, so winning can be next to impossible on a small budget. 1450 trucks make great play/baja adventure trucks and all it takes to win most races is a reliable truck that you know well.
For a ground-up 1450 build, get a 4.0 extra-cab beamed Ranger, keep it leafsprung, Giant 64s are pretty inexpensive and are insanely easy to set-up, they will be cheaper and easier than linking it, and they work better than most link set-ups on average 1450 builds. If you're looking to buy a truck, buy the Squeaker, on paper it's really similar to many other trucks in about the same price range, but it's hard to argue with it's track record.