I wouldn't be particularly concerned that the company is no longer building class C's. The chassis is standard, serviced the same as any other Ford or whatever. The appliances and so forth similarly are made by other companies, the same makes as most any other motorhome, and parts are equally available (or unavailable). There are relatively few things that would be unique to Gulfstream; and even if they still made class C's, there's absolutely no guarantee they'd still have any unique parts still available after nearly twenty years.
I looked at a Gulfstream Ultra when I was shopping for my motorhome. I really liked the layout of that particular unit. I did not like that it had sat without maintenance for at least a few years and so was in very poor shape with extensive water damage, to the point that I didn't feel safe examining the roof for fear of falling through.
