My 300SDL has 254K on the original tranny and has no signs of slowing down.

As long as they are maintained (fluid & filter every 30k) they will last a very long time.

If your fluid smells burnt, looks burnt or is milky color, dont change it... some may argue this but changing the fluid can actually ruin your tranny. If it looks and smells normal.. change it, and keep doing it every 30k.

Are you having any shifting problems?

I know of a place where you can get a rebuilt trans for that car for $895 with a 2 year warranty.