I've got an 08 KLR and just got back from a 5k mile TAT trip. I know nothing at all about BMW so I'm just giving my experience. I had to carry spare tubes. No problems at all finding 21" front tires. The 17" rear isn't as popular as an 18" but they are around enough that I don't think you'd have problems finding one.
Chain & sprockets are 520 so should be easy to find chain anywhere. Stick with stock sprockets and you should get over 10k miles easy. Changes in altitude and grades of fuel don't effect how it runs. My home is at about 700ft elevation and hour highest was around 12,600ft. My riding partners XR ran like crap everywhere except for 4-7k range.
There is also quite a bit of aftermark parts for the KLR. I have a Corbin seat, engine guards, highway pegs plus you can get all sorts of racks and other bits for the KLR.
You will need to beef up the suspension. I'm 240lbs plus I had heavy gear so I changed the shock spring to hold everything up.
The inmate formally know as "SVTA"
"If your wiener is happy, then you're happy"-Beavis
where I get my parts: www.wickedthunder.com