Fit is probably the most important aspect of any saddle. An expensive saddle that doesn’t fit is a less desirable saddle for that horse at that stage of his or her training than a less expensive saddle that does fit. Used saddles can be a good choice, but only if the saddle fits both the horse and the rider. You seem fairly confident that your horse’s body shape will change as he matures, so if I were in your position, I would be looking for the saddle that is both the best fit and the least expensive, with fit being the more important. For example if you find 2 possibilities, and one fits better but is somewhat more expensive but still in your budget and the other one is less expensive but not as good a fit, I would go with the more expensive saddle, provided the more expensive one is in your budget range. I would also never buy a saddle, new or used, that I could not ride in several times before I purchased it.