Scott,
Have pretty much owned one of every aircooled BMW made since 1970 and they are hard to beat in many areas.
Yes, the gearbox is straight cut and clunky during shifts, and the flywheel is almost heavy enough to be a boat anchor, but they will last forever if you can work past the two above issues. They will never shift like a newer Jap (or BMW) bike.
The /6 versions started in 1974 and with this you get a disk front brake and more modern instruments. The /5 from 1970 to 1974 is the old fashioned looking models and still had the drum front brake.
I have a '72 R75/5 these days. Gave up wanting to go 200 mph on two weeks. Drove the autobahn's for many years and got that out of my system. Going around the town and such on what I have today is just fine. Next move is to attach a sidecar onto it and then the fun really begins!
JoeA
Edit, just looked at the Ride West site and they have some nice bikes but are a bit high on the pricing. I bought my /5 18 months ago in excellent condition with 66k miles on it for $3500. The /5 they have listed is at $5k only because its almost new at 23k miles.
A nice one they have is the R100S for $6500 but even at that its over priced.
Here is a very nice R90/6 that should go for under $4k on Ebay:
http://cgi.ebay.com/ebaymotors/BMW-1975-R-90-6-NO-RESERVE_W0QQitemZ4579010521QQcategoryZ49977QQrdZ1Q QcmdZViewItem
Another nice R75/6, with a buy it now price of $3.7k in Oakland:
http://cgi.ebay.com/ebaymotors/1975-BMW-R75-6_W0QQitemZ4578508245QQcategoryZ49977QQrdZ1QQcmdZV iewItem
Here is a descent R50/5 on Ebay that will probably go for under $3:
http://cgi.ebay.com/ebaymotors/1971-BMW-R500-5-Nice-Original-Low-Mileage_W0QQitemZ4579058018QQcategoryZ49977QQrdZ1Q QcmdZViewItem