hello,

i am not an expert, but have experienced complete loss of forward gear with my300E.

if you have some spare time, do some research on the B2 piston, and B2(brake) band which are both in charge of forward gear. the b2 brake band is adjustable, with different sized "dogbones" which can be ordered from your nearest MB dealer for 3-4 bucks.

MY GUESS: you mentioned in your initial post that the car has 300,000 miles on it, perhaps the B2 band has worn out, and is out of spec. if thats the case, its possible that all you would need to buy is a new larger B2 band "dogbone" . replacement is pretty simple, but will require at least a couple hours, depending on your working conditions.

good luck