The most important thing is that the saddle tree is the right size for your horse and is comfortable for him or her, and you. Absolutely, buy the horse first, then look around for a saddle that fits him (get an experienced professional to help you choose one on trial, so you can test it on your horse before you buy it) Go around to 4-H sales or other benefit sales, and you should be able to find a good-quality used saddle that fits your horse for $200 to $300.
I ride with a synthetic western saddle, and i found that they are really comfortable, and really light. They are also cheaper. I have seen them for around 100 but I have also seen them a bit more expensive. If you are just pleasure riding, I would suggest a synthetic saddle.