I hadn't noticed you were in Aussielandia. (like that?)
That's cool. One of these days I gotta go there. I have distant family near Brisbane.
As for doing western riding, you can ride "English" in a western saddle. Your horse doesn'tknow the difference. I ride dressage-ish, and often ride in a western saddle. It's just a lot more comfy for trail riding and my lease horse goes better in it, and he;s downhill , too.
Do a little research into the way Western saddles are size, as it's totally differnt for Eng. Saddles, both for fitting the horse and the rider.
After you reasearch a bit, you might find one on Ebay,. If in doubt, go on the wider side because you can always pad it out, but too narrow and you're screwed.
I am not fond of downhill horses. I think that trait is appreciated when one is working cows or doing barrells as it make the horse quick on the turns, and the hind end can be very powerful in a quick start. (hence the QH being the quickest horse out there for the quarter mile)
If you can ride her up hills and ride her over cavalettis. This helps her reach under and develop the abdominal muscles, necessary for lifting the back and tucking under.
She's a cute horse and I bet very athletic.