1992 400E 3rd gear fails to engage

[glpro160]

Dear members,

I am considering buying a 400E MY1992. It looks solid and close to original state to me though I haven't got chance to actually drive it yet. The one thing that concerns me is that, as told by the current owner, its AT's 3rd gear fails to engage. That started to happen recently after the car was changed with new clutch disc.

Some mechanics tell me that AT is difficult to repair given expertise available in Vietnam (hopefully you know where Vietnam is) The problem may be resolved somehow but it will likely return. The best solution is to have a new gearbox.

So, I have 2 questions:

1. Are all W124's AT interchangeable, meaning I can swap 400E AT to any W124 AT?

2. Could this problem come from what's so called gear start relay K29/1. The relay is responsible for rapid heating of the catalyst.

I'm sorry if a thread like this already exists and looking forward to hearing response from you.