Advice
Personally, I would say - go for it. You can never guarantee what you are going to get with a secondhand bike (or car) but from my experience BMW bikes are very capable of high mileages if looked after. I changed from a 04 Yamaha R1 (underseat pipe model - nice bike) to a 1999 R1150GS last December and don't regret it at all. GS had 51,000 miles on the clock and came from Fleebay for a little less than the one you are interested in. It has now done 64,500 miles and apart from oil and filter changes and valve clearance checks (20/50 mineral oil done every 4,000 although probably not necesssary until the recommended 6,000) plus gearbox/axle oil changes, it has required nothing more than a new set of tyres and half a litre of oil between changes, must be just about run in now I guess! I have done 2 trips to France/Belgium/Holland, 2 trips to Italy and a nice long one down to Kosovo where I am currently working. Also went from here up to Brno, Czech Republic to watch the MotoGP last month. It purrs along (has a Remus Revolution exhaust and Y piece - obtained second hand from UKGS'er 'for sale' site at very reasonable prices) happily at 90 MPH all day at which speed it will do about 160 miles before the fuel light comes on, I then usually put 17 litres in, so there is plenty left in the tank. Get off the motorways and keep it under 70 MPH and I usually get 200 miles before the light comes on. I actually find that on long journeys I get from A to B as quickly as I did on the R1 as I was stopping every 100 to 120 miles to fill up on that (yes, I know it only has an 18 ltr tank compared to the 22 ltr on the BMW, but refuelling takes time). All round I think the R1150GS is a great bike, the only downsides for me are its height and weight. I have short legs and when fully loaded up have twice dropped it at a standstill when I have put my foot down on an adverse camber to find that it is further to the road than my little legs will reach, its then too heavy to pull back upright. Rocker protectors and BMW Systems panniers have prevented any real damage and I guess that I could get a replacement lower seat. I love my 1150 but it is probably going to have to go, why, because I have become a GS convert and am now saving my pennies for an R1200GS.
My advice - go for it, I certainly haven't regretted getting a high miler at a reasonably low cost.