Saddle fit and conformation/posture aside (which are obviously important), are you sure that horse is actually 15hh? I am just over 5'7'' and ride a fine 14.2hh Connemara (with a LHC) and even in a dressage saddle with long stirrups I look smaller on him than you do on that horse. I have a long body but proportionally don't look as 'big' as you do in the picture, and the horse you are riding appears to be a lot chunkier than my 14.2hh. I may be entirely wrong, but I have seen a lot of horses for sale advertised as being heights that are way off the reality. If the horse is significantly smaller then that may also affect the ease with which you could get a saddle to fit you both.
After looking at a couple when I was last buying I ended up buying a measuring stick and taking it with me to viewings as I was fed up with the horses height turning out to be vastly different to that advertised (I needed the horse to 'measure in' for the breed so realise I was having to be picky)!
Good luck with horse hunting, and I hope that you end up with the perfect one for you