If you think you want disc brakes, get cable guides instead of stops and run full length cable housing to your rear brake until you switch. As far as other braze ons, get everything you think you may need. It's better to have an unused braze on than to have to repaint the bike after adding one later. Most braze ons are very inexpensive to add at the time of the build, but can be very expensive if it includes disassembling the bike and re-painting it.