If going 1100 I'd recommend 97-99 models, mainly gearbox made more reliable and anodized wheels and a bit reduced oil consumption (changed piston design) on 98 and 99 models. Couple of other issues fixed too.
Fuel consumption: it's very much in correlation with your right hand
I'v found If you have lambda-sensored version of 1100 (so it automatically compensates the fuel mixture), then K&N filter reduces fuel consumption about 0.2L to 0.3L per 100km if running on stock exhaust, while free flowing aftermarket exhaust usually grows the consumption a bit. I wouldn't recommend running K&N in a very dusty conditions (I.e. offroad and sand riding) - independent tests have proven it's not up for the job. It's OK for normal tar-riding.
The best I've got is 4L per 100km (25km/l) if going slow 70-80kph on limited Turkish roads. On normal speeds (90-110km/h) then 4.5-5.5L per 100km. If running stock air-filter and stock exhaust, I've found the spec is precisely what BMW states: 4.6L @ constant 90km/h, 5.9L @ constant 120km/h. They haven't said anything about city riding, but it's around 5-6L per 100km there.
The worst I've got is riding almost full day 150-170km/h on autobahn and 7-7.5L per 100km, which I reckon is very low for such an unaerodynamical bike. At 150-160km/h, two up full of luggage the 1100 cruises sooo smoothly @5K rpm w/o any stress, vibrations and oil consumption and there's more power to accelerate more. It gets rough after 180km/h, where you feel you're pushing it too much (close to the top speed and redline). I've found it's very hard to make it drink more than 9L per 100km, even if riding serious offroad (as much as it's possible with such a big bike, much smaller and lighter KTM 640 offroad bikes take twice as that, some even more
). So all in all I reckon it's a very economical bike per it's CC and last century date, perfect "intercontinental-missle" bike to travel through the fast roads of Europe into the third world where you need to go slowly and where you'll very much prefer the very torquey R1xx0 engine (less frequent gear changing and less revving and stressing when the bike is full of luggage = much more comfortible going compared to a smaller, less torquey bike).
Standard 1150 and 1100 have exacly the same subframe (the 1150 Adventure has different!), so the frames+boxes are interchangable. But as the Celt says 1150 has different exhaust, so if you have the special exhaust-cutout version of the ZEGA frame and boxes, then you might have mounting difficulties. Standard ZEGA asymmetrical 35/41L set shoud fit OK on both.
Ride safe, Margus