-
Id avoid shimano disc calipers tbh. Never seen great reviews. scoble vouches for bb7s and 95% of people do well with spyres. I've got cable spyres and theyre great but never used any other cable discs. If you have lots to spend then maybe try some of those halfway hydraulic calipers.
For rotors you should only use Shimano. Those fancy ice tec ones if you can afford it/will ever do a mountain descent, for commuting these will be fine:
http://www.chainreactioncycles.com/mobile/shimano-xt-saint-rt81-ice-tech-cl-disc-rotor/rp-prod67209
1) Frames dont know the difference between discs, as long as youre not using 180s or smth too big
2) It'll only come with the end caps you order I imagine, but spares are available if you ever need them
3) You'll need discs and whatnot, but thats all you need hub wise.
Definitely recommend going compressionless from the off. What brakes are you thinking?