You have not mentioned if you are a member of the Airheads club, a national organization that caters only to the Airhead model of BMW motorcycles. (
www.Airheads.org) We have a pretty active SF Bay Area group, 3-5 tech sessions a year locally to show newbies how to service their bike, sometimes taking on pretty big projects that are completed that day so the owner can ride home,..... all while we are eating, drinking beer and tire kicking.
The monthly national newsletter has numerous vendors that specialize in parts, new and used, and services just for Airheads. For the most part, BMW dealers no longer want to deal with the older bikes. ...and you really get butt reamed on parts...if they stock them at all.
In the east bay we have monthly "Barley Therapy's" where we straighten out the problems of the world over beer and burgers. We have 5-8 rides/campouts all over the state yearly. We are devoted because the bikes really are well built and for the most part are dead simple to work on and keep running. (Think Briggs and Stratton!) Most of our bikes have well over 100K miles on the clock, some 200-400K! Get plugged into the Airheads and the local Airhead community and things will get cheaper for you. Your 800cc model is likely the best of the best models for longevity, almost un-killable! Cut out the gas tank flapper/neck and gain another quart or more of fuel capacity.
Oh and just for the record, the most expensive bike(s) you can buy are the ones you got for cheap and old BMW's are certainly no exception. If you need a lot of parts, you will certainly need a lot of money. Better to spend more and get more bike to start with. BUT the bikes that have far and away have cost me the --least-- to own have also been BMW's. I have had three Airhead GS's. I buy a barely used one for $3-4K, ride it for 5 or 25 years and put 100K+ on the clock, do all my own service on it and then sell it for more than what I paid for it. That is a cheap bike!