I recently picked up this car with 214k miles on it for under 1000.

Mechanically it's in nice shape excpet for the transmission slips like mad.

My indie shop recommended a replacement and said it would be b/w 2000 - 2200.

I found a mercedes salvage yard locally that has a "clean" used transmission with 90,000 miles on it, the guy sales it's in great shape. Tranny 750, installation 400, $1150 total for the job.

What should be cautious of with a used transmission like this?

I'm a newbie so any advice would be much appreciated.