You’ll probably need a long cage rear mech to cope with the triple chainset – depends a bit on what cassette you put on there too, if it’s a MTB style wide range one you’ll definitely need long cage.
Shifters: some are double specific, some are triple specific, some are interchangeable so make sure you get the right ones for the chainset. And unless you’re going really cheap, most grouspets now will be 10sp so make sure you get a 10sp rear mech.
Cantis: Tektro, Shimano and Avid all do good cantis, I’ve got some Avid Shorty 7’s on my CX. Just don’t expect to be able to stop on a dime like you can with hydraulic discs. The brakes work fine but you might need to alter your expectations of what stopping is about. 😉
I’ve got plenty of spare MTB parts, nothing S/H for CX though… 🙁