first get a new headunit, from there go with new speakers in front, the adapters work great to fit 5 1/4". as for what brand speakers i like the infinity kappa's and kappa perfect components. you can find them cheap on ebay, around $120 for kappa perfect components. when going with all new speakers you might as well upgrade the speaker wires to a larger gauge.
what is this "sweet 12" sub" you have? do you know its power ratings, what is the rms wattage?