I once had a Devon 2000. I really liked it, and it was definitely a quality-made saddle. Although, the "hard, flat seat" description is fairly accurate. I had been riding in a 15 year old Crosby Prix des Nations, so the hard, flat seat was what I was used to! I loved it! If you've been riding in a really cushy AP saddle, then the Devon 2000 might be a little of a tush-shock for you. But if you're used to a close contact, it should be all right. It looks like this saddle is fairly new, so the seat padding is probably not worn down either. My Devon had "been around the block," so its seat was probably harder than normal.
As for fitting your horse, I have no clue. My horse is happy in any quality-made Medium tree, so of course, the Devon fit him fine. Is the seller local, can you try a fitting?
Long story short - The Devon 2000 is a great Eq or Hunter saddle, will last forever with quality care, but might not be your 8-hour trail ride saddle. =)
Monte - 15 year old Paint/TB
Auric - 13 year old Arab