Or has been rebuilt by someone like Steve Schrimiger or Steptoe! 11's will self price once down to the sub £2500 level. You will know all you need to know by the start up and test ride as the engine seems to run smoother and smoother with miles.
The "short engine" on 11's is very strong and as long as oil consumtion is low, thwre is no smoke after possibly a slight bit at start up, and there are no undue rattles, you should be OK.
The gearbox issue applies to M94 gearboxes. Read in depth account here if you fancy...
http://www.largiader.com/tech/oiltrans/
Steve Schrim or Steptoe will be able to give you a good guide of why also.
Some of the best 11GS's I have ridden have been the ones with the miles and mine have all improved with more miles. Obvious things like clutch change, bevel pivot bearings, tyres, brake discs etc will make the cash difference on whether it's worth it but it sounds good.
Jeez, at sub £2000 even the tax is negotiable

Just think of the looks you will get from the 12 brigade for 100k+...
PS My gearbox was a M94, needed a rebuild, (quite a few don't) wasn't cheap but is now done. My 11 will stay with me long term and if you read posts from the 11 boys you will get the picture why