American Coach has a reputation for making very nice, high-end coaches. The Spartan chassis with 400 ISL Cummins is a very popular and reliable combination used by a number of coach manufacturers.
The key is to determine if the proper maintenance has been done on the drive train. If so, that combination should be pretty bullet-proof.
If the coach is in good condition and has a floor plan you like, just make sure you inspect it well for any hidden issues....water leaks, delamination of sidewalls, etc. If the coach has original tires, they should surely be replaced before the sale or a proper allowance be made in the selling price so you can get new tires.
We just traded coaches and found that it was generally more easy to get a good deal on DP's listing for more than $100K than those selling for less (in the current economy). I would suggest you should be able to buy most any coach in that price range for something between NADA 'low retail' and wholesale. If you are trading, the only real figure that matters is the difference paid. Obviously, if you trade against a low price, you will get a lower trade.