yshahar,

Check the air line from the intake manifold to the to the ALDA. If this line leaks, the engine won't have much power.

If the above doesn't fix, check the fuel filters including the fuel tank strainer.

If the selled is a MB mechanic and knows his stuff, have him fix it before U buy it. He may not know as much "stuff" as he thinks he does. But don't buy it until the no power problem is fixed.

P E H