Hey, bk, welcome to the forum.
Did this issue start right after you got it back from the stealership? If so I'd be breathing down their necks.
If not, what work did they do? Stuck in park sounds like a liquid spill in the console or leaking connector, neither of which is really part of the transmission. If they actually tore into it to replace parts they would have changed the fluid at that time but whatever they did should be itemized on the work order.
I'm not quite as jaded as ohlord about dealers, but would prefer a good indie over a dealer any day of the week.
As ohlord notes, if it's not too far gone already, changing the fluid and filter may indeed be a fix for you. For that one in your neck of the woods it shouldn't be very expensive, perhaps $250 or so. Yours has a drain plug in the converter so almost all the fluid can be swapped pretty simply, but the shop does have to be familiar with MB cars and use the proper fluid, etc.
On the broader scope, if your dad has the service receipts look back and see what has been done. By this time (assuming other things have been done along the way) plugs, fuel filter, shocks, O2 sensors should all have been changed out. Coolant and brake fluid probably need to be changed, same with the P/S and differential fluids. Letting this stuff go makes for a very large bill and since your dad let it go it leaves you stuck with it.
Hopefully you got the car for a very good price.
Take care and enjoy the ride,
Greg