Someone is conveniently located thats selling an automatic transmission that will fit my 1985 300TD. (w123).

I'm told to be cautious with this dealer. I plan on having someone else install it, I just want to buy it off him. Hes asking $500.

By visual inspection for an amateur like me how can I be sure its going to fit my car?

What models of Deisel MBs are compatible (so I can ask him what it came out of) and how could I see when looking at just a transmission, (serial no. or something) that it will be the right one and hes not just selling me some completely incompatible tranny.

Thanks for any of your wisdom.
M