if you want the cheapest reliable solution put on a single speed freewheel and be done with it.

If you want a reliable fixed gear, spend the money for a good rear wheel. If you plan to ride this bike for hours every day (which I assume most meesengers do) it doesn't pay to cheap out too much. You don't need top of the line necessarily, but you do need to spend a few bucks to get good reliable stuff. Broken bike=lost time=lost income. No?