W124 Transmission Trouble
 

I just purchased a 1986 300E (W124) at the local auction. The problem is that it only drives in first gear and reverse (does not shift up to second, third and fourth). The tranny fluid was down about 5 quarts when I started the vehicle and it was brown/black color. I warmed the car up and added the fluid before I even moved the car. I would like to know whether flushing the trasmission and adding new fluid might help (cost $100) or if I should not bother with that and simply find a used tranny (cost $500).

I appreciate any help that I could get!!