The big twins will return massive mileages if ridden with just a little mechanical sympathy and regular routine maintenance. I had an airhead RS100, it was replaced by my 1150 Adv - it had 337,000 miles on it when I sold it and it still fetched £1500. The heads had never been off, just routine maintenance.
The Adv has now got 105,000 miles on it - still on original suspension and clutch. Others fry their clutch at under 40,000 and knacker their suspension in the same sort of mileage. A couple of months ago I managed to ride from home to the top of Denmark in one hit, bar fuel stops, just under 1200 miles. Lashing with rain and blowing a gale all across Germany and the full length of of Denmark. From the Peak District to the ferry and the first part of the continent was fine.
Things that go wrong:
Wiring loom at the headstock. It gets over tightened at the factory. Most owners have cut the ties and retidied with slightly looser routing to avoid the wires breaking (the result if it isn't done)
Fuel pipe quick detatch couplings. If they are plasdtic, replace them with the metal ones. A later 1150 should have them done, apparently the factory did use metal ones in the later runs.
Stick coils - they corrode in the cylinder head. New ones easy enough to obtain, just bloody expensive (bit someone is bound to have found an alternative source at a more realistic price by now). If / when you replace them - treat the new ones to something to prevent them corroding. Paint with Hammerite or something - there has to be a treatment / idea out there that will help extend the life.
Fuel tank overflow - keep it cleared out by blowing up it GENTLY, to keep the crud and corruption out. Too much air pressure when doing it apprently blows the pipe off inside the tank. I have never suffered this, presumably because with my mechanical sympathy hat on again, I have not used excessive air pressure.
There are loads of smart fixes for the small niggles - you will find them in THE FONT OF ALL WISDOM section, or by trawling the 1150 Oilhead section.
I wouldn't get a 1200 if I were you - unless you wear orange trousers.....
