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Honestly, go for a cantilevers mount if you're getting the frame modified for a wider range of choices, the Paul mount are unique to themselves, and the Compass mount are standard centerpull, but the limit is that the best brakes are the original Mafac, or the Compass, the other (Dia Compe etc.) are lacklustre that a good cantilever is a better option.
Tektro make a great black cantilevers (CR710) with a wide range of adjustment for little money, they can be set up powerfully especially with Koolstop/Swissstop pads.
They're €19.90 on bike24, could be cheaper if you look around.
If boss placement was no object (getting frame altered) what would be the rim brake of choice? Budget probably puts fancy centre-pulls like Paul and Compass out of the running. Would need to clear a 38c tyre/ 45mm guard.
Mini Vs?
Is for a 650b conversion so dual pivots would need to be those mega long drop ones.