Flex trees are not recommended for riders over 180# nor for horses with a dippy or sway back. They flex down toward the back, not wider side-to-side. Ralide is considered the best flex tree. Circle Y had problems with the original Flex being "defective" in some way that was never explained. They replaced some under warranty. I've seen a lot of used Circle Y Flex & Flex II saddles that have been worked on. Flex II was supposed to fix that. But I'd avoid a flex tree saddle because (aside from the tree) the leather is thin and over-oiled to make it feel broken in, and it's thin. Within a short time, many people say they can feel the tree edges or the stirrup leathers or something under their thighs, telegraphing though that soft leather, and it's uncomfortable.
With your budget, a used older leather saddle of a good brand name can be bought for $200 - $350 and that's what I'd recommend.
Horsesaddleshop.com has free templates to download for fitting horse. SQHB, regular QHB ,FQHB. The center part you cut out can be taken around to tack shops to hold up to saddles to approximate the tree size.
Fabtron and Big Horn before it was bought by American seemed to be the most highly rated synthetic western saddles 2-5 years ago., Haven't looked lately. But they might be flex trees?