Can't say I'm the expert, and I'm a bit out of touch as far as asking prices for used Beemers these days, but I've got 2- a '74 and '75 R90s. This was considered the first real "Superbike" back in the day, and it's becoming VERY collectible as evidenced by the prices I've seen lately. But if you want to have something that you can ride and enjoy, that's likely to INCREASE in value, find a decently priced R90s and buy it!!
There are many BMW-bike messageboards and mailing lists out there, I'm on the R90s Yahoogroup, but that's about it.
Bret.