Go with the Nacra mast but put new raked spreaders on it so you can start with prebend. The straight spreader bars worked O.K. with dacron (stretchy) sails but if you are going to a mylar laminate sail cloth you really need a mast with prebend in order to tune the sail properly. The sailmaker should be able to give you specifications as to how much prebend to run in the mast for the sail design.

I had great experience with Calvert sails who made a set of Pentex sails for my 5.2 (now the 'frankenboat 5.2'). Elliot Patterson has a good bit of Nacra and 18sq history too that might be worth considering. A sailmaker should be a great resource for this line of questioning too...consider that when choosing sailmakers.