Your build sounds pretty good. I would change out a few parts but that is my personal preference. First, V brakes vs cantilever. There is a reason that cantilever fell out of fashion. For seat post, that cheapie that Riv usually sells with their frames is just fine. I got it with my Rivendell and it works. Bottom bracket would be a Shimano cartridge, as long as it fits those cranks. Which brings up, why so short on the crank length? Unless your really short, then by all means.
Also will you need a Wolf Tooth Road Link or Goat to get to the 42 with the derailleur? If you do, they are 10 and 11 speed and and don't know if this is feasible with your proposed 9 speed but someone can correct me if I am wrong. I am out of the loop on large cassettes.
Anyways, good luck on your future build.