It is very important that the saddle fit the horse and then fit you. The horse's comfort is the priority. So, if you don't own a horse, you really don't need to own a saddle. I have two adjustable tree saddles, because I ride lesson horses that take different size saddles, and I also have a treeless saddle and a custom saddle for my horse.
Some treeless saddles are not designed to be used for more than one horse, like Lazer saddles. The saddle is to mold to the horse it was purchased for.
You do NOT want to slap the same saddle on 3 or 4 different horses. The chances of that saddle fitting numerous horses is slim. Think of saddles like shoes. Do your father's shoes fit you? Would you want to run and perform in his shoes? You can stuff socks in there to 'make them fit' but all that chafing is the exact same thing that a saddle does to a horse's back. A poorly fitting saddle causes gait abnormalities, nerve damage, bad attitudes, and generally a bad time.
I went with my older sister to test ride a horse that I had ridden before (in one of my adjustable tree saddles).. The trainer was one of those 'old school' pad it/make it fit riders. The horse was lame. I said, let's try my saddle (which didn't fit my sister, who is 5 feet tall and a size 2. I'm 5' 9" and a size 12). The horse wasn't lame in my saddle.
So, if you want to buy a saddle for one horse, learn how to fit a saddle. I like to take 3 pieces of cut coat hanger wire. One piece I bend over the horse's withers. One piece I run along the top line of the wither down the back to the second from last rib, and one I run across the back at the second to last rib. Then, I take a saddle I think may fit and turn it upside down and put my wire model in it. That STILL doesn't mean it fits the horse, though, because the points of the tree may be too forward or too long, the gullet may be too narrow or too wide.......and you just can't tell without putting it on the horse and moving the horse thru it's paces.
BUT, you're still not safe, because your horse may gain or lose weight or muscle development and need a new saddle - again. Or, at the least, need to have the tree adjusted and panels reflocked.
Then, the saddle needs to fit you. I'm hard to fit. I ride dressage and have a long leg for my height (33'' inseam) and I don't bend my leg much when I ride. I also need a narrow twist. Saddles were never comfortable and I didn't know that I wasn't supposed to hurt while riding. I got a custom to my horse and custom to me saddle - and it improved my riding DRAMATICALLY! Riding in it is effortless and I don't even remember there IS a saddle between me and the horse. I never lose my stirrups in this saddle and I stuck 8 bucks off my 17 hh Trakehner mare when she was still very green. (Let me tell you how I LOVE my saddle.)
Also, remember that your LIFE hangs in the balance. A cheap saddle might be ok, but....what if it fails at a critical moment?
Also, let me tell you that after TWO years of research and testing saddles, I decided I was going to get a custom saddle from one of two saddle makers - and had the saddles in the correct size without custom adjustments sent to me to try on my horse. I was 99.9% sure which saddle I wanted at that point. Then, I rode my horse in them - and bought the saddle that was my second choice.........until I rode in it. The problem with my first choice saddle - the twist was too wide for me - and I had no security. While it felt good in the store on the dummy horse - I had no seat on a real horse with it.
Sorry, Erin, but there is not an easy answer to your question. The good news is that you are asking questions and with education you will make a good decision. Research, test ride on the horse you want the saddle for, research some more.....test ride some more and good luck!