I currently have 3 airheads and have owned and ridden quite a few more so I will touch on a few of the differences in models and years.
If you are going to ride a lot I would go with 1981 and up. Nikasil cylinder coating instead of iron liners, heads more tolerant of unleaded fuel, upgraded transmission with skeletonized (lighter) clutch, dual disc front brakes; electronic ignition all make a more rider friendly package.
1985 was the beginning of the monoshock on the streetbikes (R80GS had it in 1981) and this is a handling and rear tire removal improvement. Also a different front fork design made it a little quicker steering. The R80s from this era are nice and smooth for comfortable cruising but I like the extra power of the R100 that came back out in 1988.
If you aren't going to ride a lot or are going to ride primarily shorter distances or at a more relaxed pace, it's hard to beat the smaller displacement bikes. The R60/6 from 1974-76 is about as smooth as it gets in an airhead twin and there is a world of parts interchangeability.
The /5 and /6 series have a more classic look that I like but lack some of the "upgrades" I mentioned on the 1981-up models. Of course there are some of these running around with a ton of trouble free miles and the routine maintenance of these bikes is not time consuming or difficult.
The good thing is you almost can't go wrong. (IMO of course
) Check out the IBMWR marketplace and educate yourself on prices and models. Wait til winter for the best deals. Look around for a bike needing some TLC and you can save some serious $. For a while my friend and I were posting a notice in a large city newspaper to buy BMW bikes any condition and we picked up around 30 in a couple of years, some for as low as $150. Some I parted out, some I got running and sold complete.
Good luck. Keep us posted and if I think of anything else that might be helpful I'll post it.