69bbc - I own the little brother of the 396xl which is in my signature. I am very satisfied with the quality of the chassis and house. Like Gary said, the chassis is Freightliner XC and can be serviced by many facilities around the country. The house components are standard as well and parts are available by the respective manufacturers. I need to say in my 1+ years of ownership I've had very few issues with the MH.
There doesn't appear to be much that I have found that is proprietary to Airstream. Airstream may be stamped or printed on some of the components, but they usually have exact replacements by the original manufacturers. I have all the original manuals that came with my MH and have also found those and much more online. I even found complete assembly and component parts manuals for the complete unit including body panels, windows, decals, cabinets, cabinet hardware, you name it, all with AS part numbers.
I have to agree with jackiemac about Airforums, many know it all's and not too many real solutions. That site is geared more for folks with their high priced trailers, grey poupon and poodles. As far as real solutions, I've found that this site is the most knowledgeable, friendliest and the BEST one out there.
If the floor plan works for you and the MH is in good condition, I wouldn't shy away from it.
Ed