There are many nice used coaches out there. You just have to look and have some mechanical knowledge on what to look for. But at the same time I have seen some top brand coaches that have been trash. Some smell so bad from previous owners that smoked heavy that the whole coach is contaminated. But there are also some coaches that have had some TLC.
Most of your diesel pushers are pretty good coaches as far as chassis is concerned. Most have the same engines Which are either Cummins,Detroit Diesel, Cat or Mercedes. They are all good engines. The transmissions most of them are Allison six speed, which are very good.
What separates the high end coaches or flag ship coaches of every company are in the house part. The interior of a high end coach has quality cabinets, floors and counter tops. Just about every company has had different level of quality coaches. The more quality interiors, the more they cost.
I believe for part time RVing what you are looking for is about the right length 38' to 40'. If you are going full time, then I would definitely go 45'.