My advice is from my own experience - I bought a 1990 300E that was in good shape. It has remained a good car in my eyes (but not my wife's), needing only to have annoying oil leaks fixed and cosmetic touch-up. Fixing things like oil leaks is straightforward since oil can only come from just so many places. Not so emissions, An emissions system contains lots of expensive parts that have little or no diagnostics other than to replace them and hope for the best. The emissions computer will tell you what measurement is out of whack but then good luck choosing the correct part (one of usually seven choices). I know as I've spent the last year re-working the emissions system on an 83 Buick.
I would bet that after spending money to fix the emissions - considering your time, your original investment and the aggravation, you'll be missing your original $500. If it were cheap to fix, the existing owner would have already made the investment.

A bargain would be starting off with a good car and then driving the wheels off of it without it costing you a fortune. Don't start off already in-the-hole.