If you have a CEL, pull the codes. They might point to their own expensive repairs. Cruise it by an autozone or Aamco, they're read codes for free, get the codes and then figure them out (don't just take their translation/diagnosis). If it's a MAF or O2 sensor and it's been going on a long time, you can figure the cats are probably ruined as well.

Plus three on a good PPI.

And the old saying quoted here about cheap benzes is far too true. In the end you might end up spending ten thousand on a seven thousand dollar car.

Good luck.