With the money you will save by doing the job yourself, I would recommend buying a dipstick for the trans so you know exactly how much you have/need to add. I recently did this same service on our E320 and as suggested above, replaced the connector, replaced the tranny seal, tranny filter, and fluid. It was under $100 said and done and now the tranny service is out of the way.

I bought my dipstick on ebay for ~$25 shipped, a small price to pay to ensure the $3,000 tranny is good to go. I tried Woody's method and I was much more comfortable with a purpose built tool then a zip tie...