I can only comment on pricing. To start, a "good" used grill, that is all parts present, everything functions as it should, structurally sound-think rust, parts still available, the general overall look would indicate
how it was cared for, etc; @ best the price should be no more than 1/2 of pre tax retail of the pit.
Just like an automobile the price deprecates with age. Even though an auto is in top shape with low miles, time takes its toll on value unless it's something special..The same goes for pits.
The new ones may have more and better feature and a warranty. As long as what you are looking at meets all your needs you might consider it. My guess would be in the range of 1/4 the original pre tax price.
Just remembered ; Some of the GMG had augers that warped or jammed up.. Just make sure. Give it a good test .
As is said, "good luck with that".