TP,
Find out where the problem is first.
If it is the radiator, fix the radiator, sniff test the trans, then either fill (if burnt) or fill then flush the trans (if clean), if you are still worried or it's burnt, sell the car, ASAP.
If it's the transmission (or radiator and transmission):
If this is rear wheel drive, you can do it yourself if you have some mechanical ability. Pick one up from a salvage yard (they come with a short warranty) and put it in with some help. BEWARE the torque converter, if you install it wrong it will shatter the pump, it should pop into place 2-3 times depending on the model (turn and push, turn and push, turn and push, no forcing). If it was not removed, ensure it's still all the way in, don't pull the transmission into place with the bolts and crush things. Depending on the vehicle, you can often do this for a few hundred bucks and install it in an afternoon or weekend with fairly common tools, just make sure it's an identical match, and I do mean IDENTICAL. With today's computer controls, you have to be really careful about mixing and matching. A minor mid year change can create issues with compatibility and make it not want to shift.
If it's front wheel drive, it most likely can be done topside (through the hood) with a cherry picker if you are willing to pretty much gut the engine bay, but this is usually at least a week or more project for most people and not to be taken lightly. If you want to keep this car for some reason (sentimental, collector car or something), fix it, but take your time. Now is the time to do the front suspension and add some performance parts while it's down and easy to get to. You may also want to take your time and buy a cheap commuter you can sell again later to use while it's down.
If it's not a car you want to keep, trade it in or put it on Craigslist and be done with it. There are a slew of guys willing to buy a car like that to fix and sell for profit. Expect to get about half normal value, but that may be better than putting more money into an older car that may be starting to show it's age. This is not a project you want to tackle on something you have no attachment to.