No reverse when cold + tranny out what else?

So i have the motor and tranny out of a car that blew the motor. I have a fresh motor to put in but the person I got the car from told me that the car would ONLY go into reverse after its been warm and driven around. It would not drive in reverse when cold.

I did a little research and I see people talk about the B3 piston but their symptoms seem to be the opposite. Dosnt go into reverse when warm.
Does my problem still seem like B3? Is this a job that semi mechanically inclined person can do themselves. Is there a writeup or procedure around? Where can I also buy this piston?

Also since I have the tranmission and motor out are there obvious things I should do besides a complete overhaul? Certain seals, trans filter, flex disc, etc..??

Thanks alot
92 300e 4matic