Borrow, borrow, borrow before you buy tack.
You may discover you like the two ear headstall but your mare doesn't like having her ears pulled through (some horses have sensative ears and get irritated by certain styles). If your horse is directly between bridle sizes, buy bigger. You can always add a hole or two, but you can never make the leather/nylon grow an extra inch.
You'll want a saddle at some point. You need to try out all the different styles and sizes to figure out what fits her, and what fits you, comfortably.
Do you prefer a roping saddle or a barrel saddle? They're actually different in terms of fit and comfort for you as well as her.
Full QH bars tend to be the way to go, size-wise - but pads also matter. A saddle that is a hair big may be fine with a thick pad and be more comfortable than a saddle that is a hair too small.
Reins are a personal preference thing - just make sure they're long enough