Is the waterpump weeping/leaking? If so, change immediately, and if they told you it was bad, chances are pretty good that it is bad. If you have basic mechanical skills, basic + tools & a place to work, this is a very do-able repair. Stu Olson's TJ web site has a good article giving step by step instructions on changing the waterpump, which would include changing the thermostat. Just a few days ago, on here, somebody gave very good instructions on changing out the idler/tensioner pulley. Good luck & welcome!
If you don't have the tools yet, what better excuse to get them? If you have to a shop, do some looking around. Look for a smaller independent shop, might get a better deal. Also what year, engine, trans do you have?