You will not find any new saddles in your price range that are worth buying, and not a lot of used ones either. It would help if you could save up another $200 and have a $500 budget. But, I certainly can understand if you can't
It sounds like you would fit in a 17" seat. You basically add 1.5-2" to your western size and that should be your approximate English size.
Tree size is important too. If you have your own horse, then use your western saddle as a guide. If you horse goes in Semi-QH bars, then you'll likely need a medium or regular tree. If your horse goes in QH bars, then you'll likely need a medium-wide to wide tree. If your horse goes in Full QH bars, then you'll likely need a wide to x-wide tree. All saddle manufacturers are different though, so one brand's wide may fit, but another's medium may fit. It's best to find a shop (near you or online) that will let you try out or exchange saddles. Some will help picking the right saddle by looking at wither tracings and photos.
If you do not have your own horse, then I would suggest getting a changeable gullet saddle like a Wintec (likely in your price range) or newer Thorowgood (hard to find used, but they are reasonable new on Dover.com), or a flex tree saddle like a Courbette Magic. The Courbette flex trees will fit a medium-narrow to medium-wide horse well.