Yes, bugman, if the rig goes 750 you need brakes, just the cable type will do.
New trailer is the way to go. Cost, isn't a concern as the appearence of a new trailer under a boat for sale will enhance the saleability of the unit.
Cold gal is Ok, but please cover the whole unit in Tectyol 151 or equivelant. Doing this at new will keep the rig in tip top condition for at least 3 years when another spray coat will keep it rust free.
Don't take the chance with tricked up trailers, re-worked ones. >