I'm a fan of American Coaches in general, and hope this all checks out well for you...
I would also specifically ask for the ISX650's Serial Number, and double check the configuration with Cummins relation to what you mentioned about some ISX's having problems. Very common for a one, sometimes two, year old engine being in a coach. (We have 2004 model coach, and our engine was built in July of 2003, as an example.)
If this has the valve train that was in the problem ISX650 design range, I'd either pass, or negotiate a purchase agreement that includes an Extended Warranty that would specifically cover a catastrophic failure repair.
(MR D., whom you can find on the Country Coach Owens's group and or Cummins section of the forum - has I believe it was a 2009 with the ISX650 that did have such an engine. Cost him, even with some 'help' from Cummins - a substantial hunk of change to get his engine fixed. It does look like the Cummins repair solution is now solid, and a one time fix makes this a non problem...)
Sure not trying to say that the ISX650 in that coach is one of engines with the Valve Train problems - just saying it is worth your due diligence to know before agreeing to a final purchase price
Interesting that the Newmar felt 'different' to you. I thought it had the Comfort Drive in it? So I would have expected it to be 'different' feeling upfront, but after a longs day drive, a pleasure to have driven???
Best to you,