1.) It's subjective. people prefer what they prefer. If you asked me, I like the TJ because it is not as difficult to maintain as a JK and not as uncomfortable as a YJ or CJ. TJ's are the happy medium in that regard.
are the grand daddy of them all. They are what made Jeep Big. if its old and got round head lights and looks similar to a wrangler...its most likely a CJ.
(1989-1996) is the product of brand management by Chrysler when they bought out AMC/Eagle. They had the Jeep Brand but wanted to make it unique and Chrysler-esqu. The lowered the stance, installed track bars for some reason, offered a couple different trim levels, and made the interior far cushier than its CJ predecessor. Still capable and still Jeep, It was the first and only Wrangler to have square head lights.
(1997-2006) Is the response from Diamlar/Chrysler to calls for a more capable wrangler. Out with the leaf springs and in with the Coils. The Round head lights came back, and the interior, while even cushier than before, maintained its utility. (in 05 and 06 they offered the TJ unlimited, otherwise known as LJ with a longer wheelbase.)
(2007-present) The current model is set to be replaced in the 2016 model year. but for now its what you see everywhere. 4 doors or two. All of them had the 3.8L "minivan" engine until late 2011 when they started being fit with the new, more powerful and more economical 3.6L Pentastar V6.
3.) Common problems usually tend to depend on how you drive it. most of the newer wranglers that dont get beaten on the trails suffer from minor electrical problems but nothing terrible. but if you use it, or most of all, abuse it; broken axles, body damage
, blown tires, U-Joint failure
, Cracked windshield. all of these examples seem to transcend all the wrangler body styles from CJ-TJ. and the electrical issues seem to plague the JK's more than anything else from what I've seen.
there is no cheapest model. Be prepared to break out the Benjamin's no matter which model you go with, especially if you plan on doing some wheeling. I drive a YJ because they are tough as nails when properly maintained
, dirt simple to fix out on the trail with a basic tool box and know-how. Not to mention that plenty are for sale so you wont have a hard time finding one (or two if you decide to buy a donor for parts lol)
CJ's are the same way as the YJ's but they are becoming increasingly difficult to shop for as time goes on. TJ's and JK's are still new enough that vendors feel comfortable raping people on prices for replacement parts.
Thats just my opinion. Im sure everyone else here has completely different things to say on this issue.
Ive typed enough. sorry.
Good luck with the search and welcome to the forum!