These transmissions are easy to rebuild. The seals are just shot, and some clutches. Five thousand is a bit steep...
Lots of data here to do it your self. If you do not know how to diy or afraid. I'd buy one from one of these guys as they are the best, and cheap...

rebuilt remanufactured mercedes benz automatic transmissions

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rebuilt mercedes transmission price list

Here is a thread where their services was used.

http://www.peachparts.com/shopforum/tech-help/308281-help-stuck-road-722-3-no-3rd-4th-gear.html

Best of Luck,

MArtin