A day loaner of an 850GS persuaded me to trade in my K75S in for an 1150GS three years ago.
I loved the 850. I bought an 1150 because, at the time, the difference in new price was negligible, and the 850 was based on the 'old' 1100GS. So the extra for the 1150 seemed worth it.
The 850 is a great bike - overpriced at new, overpriced by BMW dealers (it costs as much as an 1100/1150 to make). It was made to fullfill a need in the German market (just like the old R450).
It has a smoother, free reving engine, a completely different feel to the 1150. I love it, but I have never bought one because of the difficulty of selling it second hand when it was so closely priced to the 1150 and didnt 'benefit' in resale value from either the 'update from the 1100 to the 1150', nor the rave reviews the 1100/1150 was getting in the bike press at the time.
So if you want one, buy it. But bear in mind that it will be very difficult to sell on. The price offered is what my 2001 1150 GS was supposedly worth on a 'Y' plate with 44k on the clock a year ago.
Then again, "Long Term Storage" can mean a lot of different things.....