I ride western mostly but like the feel of an english for beginners who are just finding their seat.
I know people who buy tack online. But usually just halters and bridals which have pretty standard sizing. And I wouldn't suggest it unless they allow returns. I would suggest finding a local tack shop. That way you can see and touch the items before you buy them. Plus they will have a knowledgeable staff to help you.
Fitting a saddle requires a little measuring. Here is an article on how to fit a western saddle. The people at your tack shop can also help you. Don't make the mistake of buying a saddle before you a buy a horse. Wait till you have the horse to make sure the saddle you purchase will be the right fit. Nothing worse than spending hundreds of dollars on a saddle and not having it fit. How to best fit a western saddle on your horse