I've got a Master Campdraft with horn from DownUnder. It has a shallower seat than the one you are looking at, but is still very secure feeling. If your horse spins, the poley catches your thigh and spins you with it. I ops checked that when riding a spooky horse a few weeks after badly bruising my hip (bad enough that it still is sore 2+ years later). I could barely get on the horse, but when she jumped forward and spun around, I went with her. Ended up with a bruise on my thigh, but that beat taking a tumble.
If they advise a particular model for your horse's back, listen. When bought on a discount, the saddle is decent quality for the price. I'd try to avoid paying their full price.
The leather was dry when I got it, but that happened with a new Bates I bought as well, and some time using Passier Lederbalsam has helped it a lot.
I've had no problem at all with the overgirth. I never feel it. I have mixed feelings about the horn. It is small and leans forward, and is darn near worthless for holding on if your horse bucks. OTOH, a couple of months ago, I couldn't get my mare to stop...she spun herself up and we went 2 hours without her stopping, so I got her to disengage, wrapped a loop of rein around the horn so she couldn't straighten out immediately, and got off. Quickly. I think the loop slowed her down just enough that my foot hit the ground before she bolted 50 feet. On the whole, though, if I had to do it over, I'd skip the horn.
My main dislike is the bulk of the Aussie girth under my leg. I have better contact with the horse's side with a western saddle. Still, it is a comfortable and secure saddle. Easy to post in and the seat feels more English than Western to me. I prefer the Aussie stirrup leathers to English ones, but have taken to riding mine using western stirrups (although the photo below has English leathers...need to update my pictures).