You don't need to meld anything until you get your 75 alexandrites, so you can do all your spiritbonding in how everlong it takes you to do that. If you know what kinds of materias you want for your weapon, you can sell the ones you don't need, for elemental materias and stat materias you might not want, you can either meld those to gears to increase spiritbonding speed or you can try your luck at transmuting in hopes of either getting what you are after or something that sells well. Once you get all you alexandrites, just get all your gils and materias you have amassed sit in front of the market board and infuse what you have, buy what you don't. Materia is random when you get it, but just because it isn't what you are after, doesn't mean it can't be useful in getting you what you are.