I'd get a used transmission. I remember taking my Miata to AMCO and they wanted $1500 just to pull the trans apart and see what was wrong. I went out to miata.net and bought a used one with 1/2 the miles for tons less, $250...I know how you feel, I have put $3,000 into my SDL to get it running real well and in great condition. I prob have another 1,000 more to go but hey, it was $50k+ new and it is a well built, durable vehicle that should last a long time. The benefits outweigh the cost easily.
Last of all, please don't take offense but it is nobody's fault but your own if you do not check the fluid levels on the car. The way I look at it, it's my car and my responsibility. Unfortunately, no one else is going to look out for you so you inherit the job automatically. I agree that any car that runs out of fluids can be expected to break down in no time, that's just a given, no fault of Mercedes. You know your situation better than anybody so make the decision that works best for you and not just to please others.