Please 'try before you buy'. It's all good and well to buy something online, get it home and realise that you physically cannot ride in it because it leaves you in pain, or just will absolutely not fit your horse no matter how much adjusting a qualified fitter/saddler does to it. Yes, I have been through both experiences, have had many saddles that have given me shocking hip/pelvic pain within 10minutes. And have once spent more adjusting a saddle than its purchase price, and it never ended up fitting that horse anyway.
That Ascot saddle looks cheap and nasty, I wouldn't touch it with a 40 foot barge pole. You are best to look at saddles that retail for higher prices, and buy second hands. Saddles are like cars, as soon as they leave the show room they drop rapidly in value, you can pick up a great saddle for nearly half the retail price.
The more expensive saddles are generally hand crafted and have a huge amount of research and scientific work put into getting them just right - but even then, what works for one horse won't work for the next.
- One pair of jeans might make your backside look tiny, and the next person's backside look huge. Same goes with saddles, it's not one size fits all.
As for sizing, you have quite long legs for your height, I would at least go for a 17 inch. The sizing of saddles has more to do with the length of your thigh than your weight. Dressage saddles are deeper and more straight cut than jumping and GP saddles, so generally require a slightly larger size.