I would get the horse first, for all the above reasons, of fitting the horse first, rider second. As for getting the saddle, I would definitely go for a used saddle save a few bucks, you can find some really nice ones. I do agree that I have ridden in some saddles that I would LOVE to buy, and I think would make a big improvement on my seat and the way I ride, but I just don't have the money to spend, and the horse I have now is currently not very easy to fit unfortunately, so I would like to wait until she's done growing and filling out first, and then at some point after that maybe think about getting a nicer custom fit saddle. Let your trainer know what your price range is, so they can better help you find what you need. If she's thinking that you are willing to spend 4k on a saddle, then she's going to pass up some potentially great saddles in the say 1k range because you can afford better, but if she knows that you can't afford more than X amount of money, then she can find you a great saddle in that price range and not waste your time or her time looking at stuff you can't afford.