Drafts have to be the hardest to fit properly. They can vary in size from a tiny 1400# up to 2000#+. They also have the largest range in body length, head size, bit size,.... I would never buy online a first harness unless you are buying from a true harness maker. They will need a ton of measurements from you to assure a proper fit. It's also every beneficial to be able to harness the horse with the maker/seller to make quick corrections to the fit. Never buy off eBay
. Overall it's a terrible dumping ground for inferior harness both in fit and workmanship.
Materials and style choices are rather personal. I only own leather ones but trust me, hiking a 60# harness up on an 18H horse can be a joke. Biothane or nylon can cut the weight in half. Bio can be close to the same price as leather while nylon might be half. Breast collars will be cheaper than collars and more versitile when being used on multiple horses as you don't need individually sized collars plus they just don't have as much leather so cheaper to make.
You'll be $$ ahead by finding someone close to help guide your purchase. Look into driving or draft clubs in your state. I'll admit to buying sets of harness at auctions only to get them home and they don't fit anyone or are missing important pieces. So much for the great deal...