Some of the answers depend on which saddle you bought.
I use a wither pad with my Australian style saddle. It is a more standard version with the Aussie girth. With my Arabians, the wither pad (Fleece Western Wider Wither Pad - Statelinetack.com
) gets the saddle up off their withers. I guess that means it doesn't fit just right, but the horses ride well when I use the wither pad.
Some of the versions that use a dressage girth recommend having a thicker pad under the saddle. Mine is supposed to be good with the standard cotton English saddle pad, but I prefer a Navaho blanket.
I don't see why you would need a breast collar. All the reining functions remain the same on the trail. A snaffle is still a snaffle, a sidepull halter is still a sidepull halter, and neck reining still works if the horse knows how.
Some Aussie saddles expect you to ride with feet forward, while others are designed for heels under the hip.
I like them. As best I can tell, my mare prefers riding in the Aussie, while the gelding likes the Circle Y better - although both can be ridden in either.
I like Australian stirrup leathers. I use mine with western stirrups (2.5 inch neck). They seem to swivel around less than the 1" English leathers do. The are designed to come off if your foot gets caught, and it seems to me the design might be a bit iffy for going up a steep hill - my stirrup leathers will come off very easily, much more so than on my English saddle. That is good or bad, depending...
But there is enough variation that it is a bit like asking, "What is a western saddle like?"
Posting this picture just because I like it. Notice most of the guys are riding with their feet further into the stirrup than most Americans do, and a 'chair seat' is accepted as normal. Also notice the poleys are parallel to the thigh - that is how it is intended. About a finger width of gap.
This picture is just for historical comparison with the old west (Charles Myers cutting animals out from the herd. LS Ranch, Texas