Having owned and serviced several in the last 5 years, here's some observations which are absolutely worth what you paid for them...
• Generally speaking, the engines and gearboxes are equally robust from model year 1975 to 1995. That is, a 1978 model is just as likely to have a XXX issue as a 1994 model.
• Another highly generalized statement is that the early-to-mid-80's models are possibly the best. By that time they had 90% of the upgraded refinements (dual acting brakes, easy clutch, 5-speeds, good electrics) and good build quality.
• Ride the bike at your intended speed before buying. Engine smoothness varies widely from engine to engine, and model year to model year. Some is tuning, some of it is build quality. A smooth R75 is preferred over a "buzzy" R100.
• The models with the front axle in-line with the stanchion tubes handle quicker, which is better for "sport touring" but can be a distraction on long straight highways. The earlier models with the off-set front axle are much more "neutral" and "easy going".
• The single-sided rear shock models don't seem to have held up well. If you go to any Airhead rally with 100 bikes, you'll be lucky to see 2 single-sided. I believe the design lends itself to shock and final drive repair costs that are 4X that of a dual shock.
• Remember, any Airhead is going to be a "vintage" or "classic" bike. Like any Model-T or Avanti owner, you'll be doing most of the maintenance yourself.

I'm late getting back, but thanks for your response. I'm looking forward to owning & caring for an airhead once the craziness of building a business dies down. Until then, it's good to build knowledge and awareness, appreciate your insights!

Thanks Scot for your gracious response! I do look forward to ownership when the time is right (not a budget issue at all, I just don't have time to do my own maintenance while I'm busy building a business). What keeps attracting me back to the airhead platform, particularly the S and RS, is not just the look of the bike but the way BMW packaged them--comfort, style, modest weather protection and hard, lockable storage in a light-ish and maneuverable package designed to go long distances. If the market offered anything comparable I'm sure I wouldn't be looking so hard at an airhead. But the only bikes that are somewhat comparable are all adventure-styled with oversized ammunition boxes for luggage. Not what I'm after. I want a gentleman's sport touring machine with high reliability and classic style, and I don't think anyone has made anything that is a closer fit to the ideal than those BMW airheads.

Safe riding.