I wouldn't get braze ons for caliper brakes on a disc compatible frame. You can get long reach calipers that bolt on that work really well. And if you are using tires bigger than 40mm or so, you probably want to go with disc
The braze ons for disc and caliper are so different that it will be a mess. I would go for disc in that situation. Disc is much different than caliper. You need rear disc clearance and a heavy fork. Having said that, I don't really see the need for disc on a randonneur.