If you plan on trail riding/rock crawling more than most people do, I'd look for a CJ8 "Scrambler. It has a longer wheel base and it's really a CJ7 with a pickup rear so it's a 2-seater unless you throw in a back seat. They made far fewer CJ8s, so they will cost you more money but they're worth it.
If you're going to use it as a mostly pavement pounder, I'd say get a CJ7 and if you have the cash, do a coil suspension swap and get rid of (sell) the leaf srpings.
TIP: Buying a CJ is usually a crap shoot UNLESS the previous owner really took care of her and restored her well. Things wear out and rust on CJs over the years. Be prepared to replace MANY things. Check ALL the rubber on the doors, windshield, etc. One thing (from personal experience I strongly recommend: REPLACE the front shackle hangers right away. Those things ALWAYS crack. Warrior makes a gusseted set for around $50. Do it. Or this will happen out on a trail... :hahaha:
About the engine: Two engines that the CJs came with are the straight 6 (258) or the V8 (304). Many younger guys think having a V8 in a CJ is the shiznit. You can achieve just as much torque with a 6 cylinder engine and the right gears. WHile I like the straight 6, the carb and vac systems were a nightmare. I swapped in a 4.0 from a Wrangler - it has fuel injection. Better way to go.
About the wheels: They came stock with 15x8 rims with a bolt pattern of 5 on 5.5. This is a tricky thing because Jeep has vaselated between 5 on 5, 5 on 4.5 and 5 on 5.5 - so make sure before you buy anything. I would swap out for 16" wheels. Many more tire options and aftermarket brake items available in 16" size.
About years & axles: CJ7s were made from 1976 to 1986. The latter three years were the best (in my opinion). In some earlier years and 1986, they came with Dana 44 axles. Dana 44s are a bit better than Dana 30s or 20s. Mine came with a Dana 20 in the rear but I swapped in Superior axles. The other weak point to the Dana 20 is the axle tubing wall thickness. But nothing a simple truss couldn't fix, eh? Bottom line, try to find a CJ with Dana 44 axles or at least the rear. If you need a visual on the different differentials, try Google - click on "images" and then type in the Dana 20, Dana 30, Dana 44, Dana 60, Ford 9", etc.
In my opinion, I would stay away from the CJ5 if you're only going to use it as a DD (daily driver). It's shorter wheel base gave CJs the rep for flipping and the space from seat to steering wheel is shorter - not so great for a larger guy.
CJ6s are nice, even more rare than the CJ8s - might be harder to find "SOME" parts though. Same goes for the CJ10. I don't know if you're even considering an old CJ2 or CJ3, but they're a whole other headache with parts and old technologies - BUT they look even more cool. That's why a lot of guys run with "flat fenders" on newer Jeeps to look like the older style and better clearance.
A word about YJs. 1987-1991, the Wrangler had a style change but MANY parts are the same as the later model CJs. I just don't like the square headlights and square indicators, square car-like dashboard, etc.
Remember: MANY parts from the newer Wranglers can and will work with the older CJs. I used many on my CJ.
BUYING TIPS: When buying a CJ, I can't impress upon you enough...bring a MAGNET! Place the magnet on various places on the body. If it falls off...bondo (or filler) is under that area as a magnet will only stick to metal. UNLESS the Jeep has a fiberglass tub. In that case run away. Fiberglass might be lighter, but it's NOT safer and tends to crack easier while offroading. While I know that anyone who takes their rig off the pavement will probably have some body damage showing or some that has been repaired, you just don't want one that is mostly Bondo. Especially because odds are good that the frame is tweaked as well.
Also check for rust - on the frame, pull up the carpet and look at the tub (especially under the dash areas) and make sure to fold the windshield down. Look for rust under the rubber cowl and the frame. Look to see of the windshield frame is the same color as the Jeep. That's a sure sign it rolled over at some point.