As the others say, it's not much, the engine might outlive yourself if you maintain it as recommended..
Since you have not got any experience from BMW boxers (as I interpret your post) you will have more difficulty determining if engine, gearbox and final drive is OK. The beamer boxers are quite rough and clonky in comparison with other types of motors and drives.. so if you feel this, it's most likely OK.
To see if you can hustle on the price a bit, check all wear parts, check if discs are warped, check wheel bearing, AND since I noticed play in the "top yoke bearing" on my 2004 see if you can wiggle the handle bar in a manor that would make the fork tubes move up and down, the this thread: https://www.r1200gs.info/forum/2-gene...g-failure.html
Well, also check the skid plate mounting points, look for any obvious cracks in the engine/oil sump.
Check if the toolset is there and OK underneath the seat, if its not there, pay less.
=) I can't come up with anything more at the moment!
Good luck, bottom line is that these bikes are awesome and rarely ridden really hard, so age does not matter much at all.