105 brifters are compatible with "road" mechanical disc brakes but not with MTB disc brakes and are not, of course, usable with hydraulic discs.
I've built up several bikes from a bare frame/fork and a "build kit", either purchased, transferred from another bike or pieced together from my own surplus parts. If you are familiar with basic mechanics, know what parts are compatible and have basic bike tools, it's a fun and rewarding project. If you have to buy your parts at retail it is a quite expensive approach.