As someone who taken the plunge recently I'm seriously impressed with the R1100S, I've completed just over 1k on it now and just can't get enough.
My tips:
1. If you get a test ride make it a long one, if you're not used to BMW boxers they can take some getting used to, you'll probably hate the 1st hour (I did) but after that it justs get better and better

2. Avoid tatty bikes, particularly neglected and rusting ones (check the suspension, usually the first to go), better off paying more money up front than shelling out good money trying to tidy up a bad one.
3. If you're buying from a dealer make sure they throw in a full service, I checked on a quote for a 24k BMW service & it was £600+
I won't get drawn into which colours best, cos Daveg's already covered it, not that I'm biased
