Since you don't know what road you will take with your mare, as far as a discipline goes.......I would get an A/P saddle that you could easily resell. That way, you can ride all disciplines until you choose (or if you decide) to go down a one-discipline path (like dressage, or hunters, or jumpers, etc).
That way, when you do decide, you can get a saddle for that discipline and get close to what you paid for your A/P.
Having said that, I would highly recommend a saddle with a changable gullet, either a Wintec or a Colletiate. You didn't want to spend a small fortune, I don't know what your definition of a 'small fortune' is. But you can pick up a used Wintec 2000 A/P saddle for around $300-400. they are VERY comfortable and easy to take care of and really hold their value. I've had 2 Wintecs and both times I sold them for MORE than what I had paid.
Collegiate is the same deal, only leather. The problem is that you are going to be looking at $700-800 for a nice used one, and probably $800-900 for a new one (although I have not recently checked pricing on new ones). They are nice saddles, but you may not want to put that much money into a saddle right now.
The reason I am suggesting you go with a changable gullet is that you said your mare has high withers, so she is probably a narrow or medium narrow tree and those are almost IMPOSSIBLE to get rid of. I personally will never buy anything other than a WIDE ever again, because a WIDE will fit virtually any horse. But if you are trying to get rid of a narrow, there are very few horses that require a narrow, I think you might be stuck with it.
JMO. Good luck
