If it looks good then it probably is. Take a good look behind the plastics. If allowed remove the fairing (sorry, 17 bolts each side). If it's in good nick underneath then it's probably fine elsewhere.
I bought a MY 02 silver one a couple of years ago. It looked great but it had a problem with breaking final drive bearings every few thousand miles. Mine must have been the fabled Friday bike. A closer inspection of the service history showed this bike had a long problem with that bearing.
Take a good look at the documentation with your bike and see if any infrequently replaced items get replaced frequently.
Just so I'm not putting you off, my 1150GS is on it's original final drive bearing at 80,000 miles.
If the bike has been parked on the side stand for a while then don't be put off by a smokey startup. They all do it.
Some rattle for a moment or two until the oil gets everywhere especially after a few weeks laid up. Don't worry unless it doesn't stop.
The brakes are linked front to back and back to front and the servo is overpowered. A gently tap on the back brake will have you nutting the screen. Don't worry, just be aware of it.
Make sure the alternator belt has been changed recently. Service interval is 24k. Don't let anyone tell you any different. BMW reduced it from 35k a few years ago. It's a real pig to get at because of all the bodywork that needs to be remove but it's a 10 minute job after that but could knock you a few quid off the price.
Get the bike on the centre stand. Grab hold of the rear wheel at 3 and 9 oclock. Pull/push to/away to see if there is any play. If there is, get someone to press the rear brake. If the play stops you need a new final drive bearing. More £'s off.
Do the same but at 12 and 6 o clock. Any play there and you need swing arm bearings. Cheap and easy to replace but, again, more £'s off.
The brake servo's whine like an orgy of rats. That's normal.
If it looks good and rides well it's probably ok.
Hope this helps a bit.
Regards
Rob