If you’re after just brakes, then TRP Hylex brakes are a full hydraulic system fro £125 an end. Good comments from where they’re fitted to full bikes (ie Pinnacle Akrose 2).
TRP Spyres get rated on a par with BB7 Road calipers, and cost the same.
My only piece of advice (based on OEM BB5s) is DO NOT put a 140mm rotor on the back – chuffing useless.. Stick with 160mm. I can’t even lock the back wheel in the wet with slick tyres on BB5’s and 140mm rotors..
Oh, but as above, if you’re going mechanical brakes only route, buy some MTB calipers and “V” Brake drop bar levers.. Probably the cheapest and best way to fit discs to drop bars.
All IMHO of course..