Slipping when cold sounds like a clutch pack problem. Especially if its been slipping for a while. If it was me, I would find a good second hand trans and put it in.
I have the above mentioned book, it is a good read. I think the trans could be original & just ready to be replaced. Maybe you have been spending too much on the MB on the wrong things. TD's are hard to come by, especially if its a good sound one.
I dont think much of volvos.
See if you can get a low mileage trans out of a write off. Some one on here may have one.
Sounds like your mech has already extracted plenty from you.
It doesnt take $450 to diagnose a bad trans!

__________________
Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving