I wouldn't buy her any tack for at least a year, to give her time to finish filling out and muscling up some. Anything you buy today probably won't fit in 6 months, at least not a saddle. I would buy a good training surcingle and start with that on her back and a saddle pad, and I'd start bitting her up for 15 mins or so at a time, just letting her carry the bit in her mouth. I also start them bitted kind of low rather than where the bit should sit, so they learn to pick it up and carry it themselves. Once she's had a couple weeks of carrying the bit around, I'd start using a 'side rein' on one side and then the other so she teaches herself about how to get release from the bit by dropping her head, rather than flinging it up. Then I'd start doing all the ground work to get her started under saddle, until late this coming fall. If she's gained a bunch of muscle and bulked up by then, I'd try saddling her and if she's okay with that, backing her just for short periods of time until next spring when I'd start putting miles on her. You can have her ready to accept a rider from right after you get her to accept a saddle (hint, buy an old beater saddle and pad it up so that if it doesn't fit real well it won't hurt, but if she rolls with it she won't kill a new saddle) by using a mounting block and gradually putting weight in the stirrup.