Fluids mixed, is the transmission shot?

The car in question is a 2003 Dodge SX (Neon), with a 2 litre engine and about 100,000km. After having not driven the car for a year (except for running the engine periodically, and driving it up and down the driveway to keep parts from seizing up), I took it back out on the road. It ran fine on the 5 minutes of side roads, but after about a minute of highway driving I noticed smoke coming out behind me. As I slowed to get onto a side road, I felt the car lose drive power for a second, and then re-engage. It sputtered a couple of times again as I brought it to a stop. There was smoke coming from around the back of the trany, though I'm not sure if it was an overheating trany, or the leaking fluid cooking off. When I went to do a three point turn to get the car facing the other direction, I had to depress the accelerator far down before the gear would engage (which it did with a jolt). On the road where I had done this, there were splashes of leaked fluid. I should also mention that there were no signs of leaking fluid on the driveway.

As I understand it from speaking to the mechanic, the transmission fluid cooler inside the radiator, burst. Coolant was pulled into the trany and vise-versa. They are recommending the radiator be replaced and the trany flushed. The repairs will set me back about $850. The mechanic also stated that there is a chance that the transmission may have been damaged from the coolant leak. If that is the case, the replacement for that would be more than the vehicle is worth. I've looked after this car, and don't want to loose the investment, but I'd rather not throw $850 at a broken trany. Based on the description I have presented, is there any way of knowing whether the transmission was damaged? Could it be damaged from a couple of minutes of coolant and or low transmission fluid levels?