Ok, so here we go!
Speak to the treeless saddle lady in Virginia. Her name is Paulita and she will help you loads! I ordered my treeless from her. I originally was going to buy a Barefoot, but after some more research I decided to save more and invested in the cadillac of treeless saddles: Freeform. (of course it was a demo so a bit cheaper then full priced). Try emailing or calling her. Take photos of your horses back, from behind (standing behind the horse so the butt and legs can be fully seen), and full body photos from the side. This way she can tell you how the saddle you are looking into will be on your horse!
Also, yes you need a special pad for the saddles. There are two different kinds, a HAF pad, and Equipedic pads. Usually HAF pads are cheaper, but still pretty pricey. With these pads, you get what you pay for! However, Barefoot does make it own pads. I still recommend the HAF or Equipedic. (I have a HAF pad)
Now onto the fun stuff! Try a saddle out to make sure you and your horse like it! Price out how much it would cost to have a demo saddle sent or to trailer your horse 7 hours. Which is cheaper? Do that one and try it out. Treeless saddles are like regular saddles: it might not be what you want and you won't know until you try.
Also remember that treeless saddles are quite an investment. Not only to you need the saddle, but you need fenders (leathers), usually some special stirrups, the special pad, a girth that will fit it properly, and I personally recommend a breastplate or breastcollar depending on the style you purchase. You also need a special oil to clean the saddle with. You cannot use regular saddle oil.
Now a little pricing help. When I bought my saddle, I decided to look at it as an investment, not a toy. So, to build my credit, I took out a small personal loan. Not only do I get amazing credit from automatic bill pay, but I don't have to pay the whole huge chunk of cash at once. It can be split up and it makes life a heckuva lot easier! So while it is going to take me a bit longer to pay off the equipment, it is in the long run making a positive impact on the future of my investments and in the future of my horse and I's backs! (I don't have the best back :/ my chiropractor frequently tells me to return to him sooner.....lol)
So to sum it up in a few points:
1. Talk to Paulita from treelesssaddle.com and ask her about sizing for both you and your horse
2. Demo a saddle to make sure it is right for you
3. Make this saddle an investment, not just a regular purchase
4. HAVE FUN WITH IT!
And you best make sure you tell me how it goes!