There are RVer's that take care of their RV's. Newmar has always had a good reputation as far as to quality. The chassis is a good one and 150K miles is not much for those diesel engines. Those engines with care should get you 300-500K miles easy. Same for the transmission. Now I reading where starting a diesel engine every once on a while is bad for it. I would have to disagree with that post. Any engine needs to be started, run enough to get the temperature up. It prevents seals from drying up and start leaking. Also fuel can only set for so long and then it starts going bad.
Now as to the age of the coach, AGAIN I do not see any thing wrong if well taken care off and if it is properly priced.
Now I know many RVer's that have bought older diesel coaches and are completely happy. Not only getting a coach that has had TLC, but they did not have to use a lot of their savings to pay for a motor home that costs more then some houses, and that lose half of their value as soon as they drive off the lot.
So I would check this coach real good, and if I needed help on the mechanical aspect, get a professional to check it out for you, even if you have to pay him a couple of hours pay.