I think you should be able to find a nice saddle, used, for that price range. And I believe you can probably use it on both horses, with different saddle pads or blanket combinations. The western saddle tree spreads the rider weight above, over a bigger surface because of their tree. Exact fit is not usually such a big deal as it is with English saddle fit and their smaller tree surface on a horse. Plus you have thicker western saddle blankets and pads that help spread out the load too.
Things to consider while shopping for Arab bodies, include their shorter backs, usually withers with some height. Most Arabs I know have shorter, rounded skirt saddles, which don’t poke horse during tight turns as square skirts can. They are actually called Arab saddles or “built on an Arab tree” in their descriptions. Such models usually have a higher front gullet to fit those good withers. I want at least 2 fingers vertical above pad or blanket, before they touch saddle. More finger room is ok too. You don’t want saddle gullet touching the pad. I personally like big rings for my front cinch straps over in-skirt rigging designs. Seems the in-skirt types wear out faster, need repair to be safe as they age.
Have you visited any tack shops with used saddles? This allows you to sit on them, maybe try saddles on your clean horse. I put the saddle on horse over a clean bath towel. This lets you feel how saddle actually fits horse, no padding to cushion pressure areas as you run your hand under the saddle, between horse and saddle above. You want no pinch points, where hand can’t slide along easily, even girthed up snug, but not extremely tight. Saddle should fit his shape, lay slightly above the body. Maybe like a leather coat fits around a person? Perhaps such a tack shop has a resident saddle expert who can help with fitting the saddle. Any “tight” spot you feel will be worse with a riders weight in the saddle. Your blankets or saddle pad just is preventing abrasion of his skin, soaking up sweat. Not terribly thick in most cases, though dense is good. I look at pad or blanket washability too. Dirt on them will rub holes in the horse.
If you now ride a 16.5 seat on an English saddle, your western seat size will be much smaller. Maybe youth size at 14 or 14.5. The cantle behind you, can make a difference in fit too. The higher-rise cantle (like many barrel racing saddles) will need a slightly bigger seat size to allow comfortable shifting, adjustment, during rides, especially very long rides. That high cantle holds you more firmly in place than the older style, low cantle with a Cheyenne roll will. That lower cantle is the type of cantle you see on many show saddles. So the lower cantle allows you a comfortable, smaller seat size, while still letting you “move around” within that seat size. I ride a western, padded 14.5 or 15 inch seat, while I ride a 17 inch seat in an English saddle. I personally do not like an unpadded seat, especially for long hours of trail riding. You want to be able to sit in the saddle with your legs under you, same old head, hips, ankles in a straight line for practical equitation. Your stirrups will be adjusted longer than you use for English riding. Stirrups should swing easily so you can keep your feet/legs under your hips. You may want to ask about narrow twist saddles, which are usually more comfortable for females. At least try sitting on some, see if they feel any different, more or less comfortable to you.
On details, I want saddle strings so I can tie on a slicker, cantle bag with some lunch, water bottle, hoof pick, sunscreen. You can get strings added with a d-ring under the back Conchos if there are no strings. I am a “buckle and tie” person about girths. I do put the buckle tongue thru the hole, but also ALWAYS use a cinch knot on my saddle ring before looping up any left over girth-latigo strap. Be SURE to recheck your girth at the 15 minute mark after getting on and then once an hour during long rides. Girths do loosen, horses dehydrate, you need to readjust cinch strap tightness as the ride continues. I also use a breastcollar, it helps hold the saddle in position over up and down hill terrain, without needing to overtighten girth to keep saddle in place.
One last tip is to keep both phone and a couple horse treats, on your person, in case you and horse get seperated somehow. Luring horse back to you with treat reward could save a lot of time trying to catch him! Phone is with you when you need it, not in the cantle bag with silly horse!
Please post back on how things are going.