I would suspect the bones of the house to be rock solid - thats a Newmar thing. Friends have a 1998 Dutch Star DP, its as straight and solid today as the day it was built.
The 150K miles would give me some pause but wouldn't make me run away. Obviously it has spent far more time on the road than in the repair shop. Miles today aren't what they used to be. Heck my Fusion has 176,000+ miles on it and I wouldn't hesitate a second heading out on a coast-to-coast trip with it.
For me it would all depend on my budget, their asking price, and the overall condition. I'd also go into it knowing it was a used unit with 150K miles on it.
As has been mentioned; Spartan, Allison, Cummins, and Newmar are all top notch builders.