In terms of what to look for  in a used one.... ?  Pivot points  ( and steering  pins) have already been mentioned, but I don't think thats a particular issue for STX/TJ's,  just any 4 WD tractor. I had a Steiger CR1225 that would  have severe wear  there in 1100 hours  but the Deere 9200 had no such problems at 4,000.  I'm unaware of it being an issue yet, at 1400 hrs on the TJ. 

The climate control has never worked worth a  damn on either mine or a friend's who has a slightly older ( mine is  an 05 build).

The 24 speed trans is  quite suitable for my application.   There is no way i can guess if it is so for yours..

I have to say there always seems to be some nuisance issues with this tractor,  arm rest controller failures ( meaning tractor shutdown), hydraulics that  actuate with no operator input, air seat collapse, alternator failure, starter failure, and the latest one is an air con that mostly won't work at all, and  nobody can find any fault.. so Ive beeen working in a balmy 92 F cab, when it's  been  60 F outside.

But i think for the most part Mike T likes his pretty well, so it's possibly a "luck of the dice roll" thing.