Rough riding tractors, hard to shift, and torque amplifier will probably soon need replacing if it hasn't been done already. That being said, they are pretty much bullet proof, and see a lot with 10-20k hours. I hesitated at replacing our TA but the dealer said once being rebuilt, they have never had one back and expects them to go another 30 years.
For shifting you just need to be patient, for ours - put throttle at 1100 rpm, push in clutch, count to 5 and it will slide ride into gear. I wouldn't want to have it as a loader tractor, but for pretty much everything else it is not a big deal. I also put an air suspension seat in and it now rides twice as nice.
Doors are hinged in front instead of back which a lot of people complain about, but never bothered me unless getting in and out constantly.
All in all, I think they are good tractors for the price they bring. I would say like any tractor - consider shape of tires, condition of cab & air conditioning, oil leaks (some are to be expected in an older tractor) but shouldn't be excessive, check bushings & spindles in front end, and whether it smokes a lot.
I made a poor purchase when I got mine, and ended up putting more in repairs than what I paid for it. But as it sits now with new front end, new TA & clutch, new air ride seat, air conditioning rebuild, new stereo, & new tires, all over 10 year period, I do not regret fixing things on it.