I Really, Really Want This Saddle and Need Some Thoughts/Opinions And I Have A Few Questions...?

I don't have a lot of $$$ to spend and I am on a strict budget and I have to buy other riding supplies as well. So I found this saddle I really want in the new Stateline Tack catalog. It is a Regency Close Contact Saddle for $349.99. The ad states 'Fine leather saddle features a soft, well-balanced seat. Includes saddle, bridle, bit, girth, quilted cotton pad, stirrup irons, and stirrup leathers. Ideal for show jumping (which is what I do)'. Comes in chestnut or havana. Seat sizes 16 1/2", 17", and 17 1/2" and comes in wide and regular. First of all, is this a good deal and a good saddle? Secondly, how do I figure out what size tree do I need? I never learned that. I also do not own my own horse and would be riding other people's horses. Thanks for your help!