chuck a 25det in with the 25det box, i wouldn't bother with straight cut gears, the 33 box is pretty strong as is. (mine handles 400rwhp without any issues)
5 stud conversion, the front hubs can be re-drilled to 5 stud, or you can get early Z31 hubs. the rear can't be re-drilled, so its either try find some early Z31 hubs again, or get some made up (probably easier to get a full set made up)
brakes wise, again i'd chuck the 33 gts-t brakes on it, you will need HR31 (import) front struts, and then the calipers/rotors are a direct fit, rear is a direct fit too. if its converted to 5 stud, everything will bolt straight up, if its still 4 stud, you just need to get blank rotors, or re-drill them to 4 stud)
diff setup, you can get a few different LSD's for them, i've got a tomei technical trax 2 way for my 31, they make them for the aussie spec as well (a mate in WA has one for sale, in the housing)