If you are going to uprate the brakes then you will need to make sure you have everything relevant for the job, I.E. caliper, carrier, caliper bolts, master cylinder, pads and disc. Tbh for your car that all seems like bit of a ball ache.
I would just get the caliper replaced, flush the brake lines, put new fluid in, and get some new front discs and pads put on. There isn't really a call for larger discs on your motor mate, if you want to make the braking more confident on it, then get some braided lines fitted, they will clear up the sponginess and the brake fade as they get hot.