Go for anything 2014 or older. The S197 generation (2005-2014) is exceptional! Especially the last few years from 2012-2014.
The SN95 generation (1994-2004), the older versions from the early mid 90's haven't aged well, but again, late in the run from 2000-2004 were pretty good.
Nothing comes to mind on what to watch out for, as my Mustang experience is personally tied to the high end GT500, so my one piece of advice wouldn't be relevant.... (there was a consistent 1st to 2nd gear grind during hard acceleration that Ford had to put out a TSB on because so many GT500's were affected, luckily it's been a virtual non-issue for me, but I have read that others have needed a lot of work related to it)... yeah, not relevant in your price range. Even if you get a S197 gen Mustang, you'll have a completely different power train.
You might end up with the dreaded "skip shift feature" though. I don't recall what year it was implemented, but if you're not aware of what it does, during certain conditions, it will close off the gates for the next gear and force you to go up another instead for fuel mileage reasons.
That's something GT500 owners never had to worry about. No forced skip shift on us... but GT and below had this "feature" forced upon their manual transmissions.