honkydory

Our 03 S500 showed a pre safe inactive warning and a check engine light together. It also would not shift from 2nd gear. It clunks forcefully into gear(rev or drive). When this first happened, the car would not start. It had power,but would not crank. then it cranked for 1/4 sec,then 1/2 sec, then finally long enough to start. Now it starts fine again, but the problem with pre-safe and the tranny persist. I've read about syncing windows but don't know how to do this. I took the car to the dealer, for diagnosis only. They claim that the transmission control unit, conductor plate, and electrical connector have failed. $$2653.00$$
I am a very capable mechanic with a background in automatic transmissions and diesels (I'm certain I could replace the hard parts)
Three questions:
Can I buy these parts?
Do these parts need "burned in" by a computer at a dealership? (Or are they "plug and play")
Is it likely to need 3 parts or just 1?? Thanks in advance.