On the newer models besides no disc brakes they also have stopped covering the lounge and dinette bases with padding and ultraleather and opted for a non-furniture quality wood front and base. Try to locate pictures. In my opinion the same fabric bases make the lounge appear more like a couch with a sleeker upgraded look and the contoured bases feel better against the back of your legs than the cushions on plain wooden boxes.
If you intend to move your Airstream, than miles of wear and tear seem to have diminished negative impact once the rock guard gets rocked and the sawdust shakes loose (beyond routine maintainence of tires and wheels and running gear etc.) In fact there appear to be more dents and scratches in the models that "stay home" than appear on the models of owners who regularly take their Airstreams with them.
The worst that could happen buying older is to trade it in and get most of what you paid for it in return but buying new and changing units should you decide to will cost dearly in depreciation even just off the lot. We have had 4 new Airstreams, and another gently used Airstream and another used unit to be bought in the works. Going backwards if you know what you are looking for and why may not be the less advantageous. I would go for the quality and condition of an individual unit and save money and frustration and not buy new again.