If you are planning on keeping the car, don't use a sealer. If you are selling it, go ahead. The sealer swells everything up inside, not just the pump seal. I have experience with this.
I used Trans-X, after about 9 months, it started leaking anyway. I was advised to change fluid several times to get as much out as possible.
My advise is, if the tranny shifts fine and isn't leaking badly yet, start saving up the $800-$900 to have it re-sealed. If it's not performing well, drive it until it breaks or until you get $2500 together for a new tranny.
One last bit of advise. I'm not one to recomend the dealer for repairs. However, Mercedes trannys are a different animal. Your local Aamco shop really isn't the place to have this done.
I paid $900 to have the dealer seal mine and two and a half years later, no leaks. Oh yeh, have the rear seal done too.
91 300E 120K
90 300SE 275K (sold)
92 BMW 525iM 120K
90 BMW 525iA 175K
85 300D 175K (sold)
84 300SD 245K (sold)