1. You can do it youtself for sure. Just find a garage where you can get the proper tools. Don't know if there are any other seals that fit, if not order the seals from any BMW dealer. Use fast DHL/Fedex if nesceserry (www.james-sherlock.co.uk
should work as well, you can use your credit card and they post it ASAP), usually few days everywhere in the World.
2. Regular fork oil. 5W or 10W according to BMW, but i've upt 15W too, not much difference for yer butt feeling since it's for lubricating only
The forks just work as an directing pipes to the telelever system, dead simple as that - thus you may basically ride forever with the leaking forks w/o difference in bike handling, just make sure there's enough oil in them to lube it's movement. But yeah, it's a pain in the arse with leaking oil to the brake calipers (security issue mainly) and to the wheel that kicks it off everywhere (making the bike miserably messy). If you're a hardcore enough you easily may as well put temporarely simple engine oil in them if it's really about "life or death" you really need to ride
3. I'm quite sure you can find fork oil everywhere in the world. Just pop into some car, spares or a scooter shop.
The best way to do it is to completely remove the forks. Note both the fork bridge (2 bolts per fork) and the upper bridge (1 nut per fork) has the bolts locktited. So don't screw them directly off - it may damage the thread, better is first to loosen them about half or one turn, then loosen/tighten with small amplitude few times, then a bit bigger amplitude, so the locktite glue cracks in the thread. Then screw them off. Clean the threads before puting on. You may tighten them w/o the Locktite thread glue too if you haven't got it there, but if you do, prefer to locktiting them again on tightening.
Inspect the dust seals as well, if they're detoriated or have any cracks/bad rubber. Indeed, i recommend buying a new pair of dust seals along with the oil seals. They don't cost much and you'll be having both potential "bottlenecks" eliminated this way, more peace in mind. Usual cause of failing oil seals are the bad dust seals that let the dust/sand inside.
Inspect throughoutly the inner tubes surface once you take them out (just pull them out after you've taken the forks off). See if there's any visible scratches or dents on the working surface, if yes, then better replace them (quite lot of £££ tho...), coz scratched/dented inner tubes will mean you'll be having often oil seal failures. Another way is to let the scratches be professionally burnished out, some offer that service, if the scratches aren't big and deep, but it gives you no guarantee, because the straightness of the inner tubes can be affected this way depening how intensively they were burnished and it means failure for the oil seals sooner or later again.
Once new seals installed - take regular care and keep the inner tubes always clean. Use soft (lightly humid, the best) cloth/tap and do not apply much force (can cause small scratches otherwise) cleaning the dirt off the tubes. It'll expand the seals lifespan considerably. Doing offroad, muddy, rainy or just dirty conditions i wipe the tubes clean almost every day, before or after the ride.
i) Here're my 1100's, taken off (the principle is exacly the same on 1150):
ii) Pull the innter tubes off.
iii) Drain the old oil
iv) Replace the seals
v) Add new oil (can't remember the amount, it was 240ml per shock? Google.com is your friend)
vi) Put tubes back (with the upper ventiles [those very small hex-bolts on the upper inner tubes] loose)
vii) Install them back to bike and on the centre stand on normal maximum position, i.e. front wheel off the ground, tighten the ventile bolts, so then the proper amount of air is left into them.
OK. That got into too long story now...
Hope this helps!
Take care and good roads, Margus