You'll just have to go look at it and see. Most of the very early CJs I've looked at in my area that were for sale were either rusted out completely or were basically made out of fiberglass and bondo. As long as the frame is solid, you can always just buy an entire tub and front clip or whatever else you may need new aftermarket. The CJ-3A will have the L-head I4 (flathead) which is a great little motor that will run forever, but if it's sat for that long you'll have to make sure it's not seized up too bad. You may have to pull the head (be careful with those bolts if they're rusty!) and douse the cylinders with PB Blaster to try and get them free if it won't move. Chances are the transmission and transfer case are going to be pretty stiff too. It really depends what you're looking for and what you want to do with it.
1986 Jeep CJ7 4.2L I6 T176 (restoration in progress)
1961 Willys 4x4 Wagon L-226 I6 T90 (restoration also in progress)