Mayur's94e300d

Hello guys.
New member here with my first post.

So I recently purchased a 94' e300d with 126000 miles on it.
Now the car is almost fine.
The issues were
1). white smoke as long as car engine is cold and occasionally it shows glow plug warning, so ordered new glow plugs.
2). Engine mounts were looking weak, so ordered those along.

I am not into mechanical stuff that much(I mean I know the stuff and works but not into core like crucial engine parts and gearbox stuff) so getting all this work done at a workshop.

Anyhow these are not my concern. The main issue which I am facing is.
The car in reverse, is not able to climb on slopes. Like the other day I was taking it out of workshop in reverse and the exit was on a small slope, so the car stopped half way and even on applying accelerator there was no movement and it was like car being in neutral. I tried to and fro movement to take it out but no good, everytime it stopped in the middle of slope.

Another thing, if it has to move over a hump from
standstill, So I have to move it forward a bit and then reverse get it going, otherwise it wont move

Each time car acts like it is in neutral and only revs rise, when this happens.

I faced the same issue in "D" only once when I was parking it and it was big hump and I started from standstill just behind the bump, so it was not able to move over it, so reversed it a bit. Back in D, gained momentum and it climbed easily.
Although everything works fine on normal surface with small bumps etc.

Another thing, the gear shifts are jerky under normal driving condition but shifts fine under spirited driving( I am yet to observe that it only happens when engine is not warmed up)

Now the workshop here are not too sure. And I asked them to change the transmission filter and oil but they are not willing to take risk. So they are only accepting on doing it on my risk i.e if the car totally stop moving post that, then they wont be responsible. These cars are rare here plus they are happy doing modern cars as parts and stuff are easily available and less risk for them.
Also the workshop that will be changing my mounts observed leakage near gearbox, which they said to check on the day(maybe tomorrow), my mounts will be changed.

But I checked the gearbox fluid through dip stick, some 10 days ago and it was fine.
I will check it again in the morning though.

Now my question is, do I need a rebuild?
Or firstly, shall I bite the bullet and get the filter and oil changed(even the previous owner recommended me to get it changed). What if it totally stops.
I am not yet able to find a workshop that can rebuild the gearbox. So I am not too sure to take that risk

Looking for you guys valuable suggestions.

Thanks