I think the overwhelming responce is going to be get an avif bb7 mechanical disk.
They are easily as powerful as a hope mono trial but near fool proff to set up and adjust, dont leak, can adjust lever feel, and you dont have to worry about pulling the lever when the wheel isnt on the bike.
I am presuming you have an xtr lever? if so, then all you need to do is buy the avid mech and a cable, that will be about £75, a lot less than your buget.
Ps get the 185mm rotor