Start with a plan and good arming garments. As Sir Ed described, your envisioned kit is not from the 13th century. So decide which century you're really after, or decide to do a fantasy kit. Either way, once you decide what type of armor you want to wear, before you purchase a single piece of it, I highly recommend squaring away the arming garments first. With bad arming garments you can have the nicest most expensive custom made harness on the planet and it will not function or be comfortable.
If you're interested in trying to be historically accurate, please ask and we will guide you properly. You may be surprised what really belongs with what from a historical standpoint. If you want to go fantasy, then the sky's the limit.