Another point... I assume you are looking for a diesel? There was a major change in "smog control" systems in diesel engines effective in January, 2007. And, there was another major change in the past few years.
IMO, the jury is still out on effect of the smog control stuff the manufacturers had to add to their engines. While it may take many years to access any changes these controls have on the engine longevity, the 2007 change seems to not have affected the engines. But, for this reason, I purchased my coach. Although it is a 2007 model, it has a 2006 engine which falls under the "old rules".
There were a string of Cummins engines that were made during this period that had a major problem. Maybe someone on this forum can tell you more than I can remember, my "oldtimers" kicking in again. I believe it had to do with an engine rod, or something like that, and Cummins only solution was to add a red light on the dash to let you know when you had the problem! I called Monaco and got my engine number and was able to confirm that my engine was not affected by this problem. Do a search on this site for Cummins recalls and see what you can find.
Other than the engine issues (affecting all manufacturers), and the R4R chassis problems, Monaco has a great reputation for building quality coaches during the time period you are looking at.