Your problem with the heater more than likely is the diverter box gate.
Mine has AC and had the same problem.
The gate in the box is plastic with a rubber seal around it. Breaks near the pivot point.
I bought a new one for about half price.
Try:
http://www.gmpartsdirect.com
http://www.partszoneonline.com
As far as the thermostat, I believe the stock is around 180 degrees...........