If you don't plan on showing, nope you don't have to change anything.
I got my saddle on craigslist for 300. But I don't know about finding one for 100 bucks.
If you have friends that ride english, ask around. A lot of times they'll have old saddles they need to sell.
And for sizing, i'd assume (guessing your age) that a 17inch would be the best fit saddle. (I'm 17 and ride in an 18 inch.) If your horse has average or skinny withers, find something with an average tree. If your horse has broad withers, go with a wide tree saddle.
You will also need a girth. (the western chinch wont work.)
For that, measure your pony once you get your saddle.
Hope this helps
Last edited by mselizabeth; 12-04-2011 at 11:08 AM.