Just thought I'd give an update.
I ended up going with my first choice of brakes, which was CSC. I'd always said that was the setup I was going to go with once I decided to upgrade, and thanks to a lot of help from Stuart at Benson Motorsport (t01-100 on here) I picked these up last night.
So now the brake setup is CSC 4 pots with 343mm discs, 33 GT-R rear brembo calipers, and a BM44 Master Cylinder with the ABS blanked out, set up for use with no booster. Should be plenty for my lil' old 180SX