Oh, I'm sure she'll get along fine with the breast collar harness until she's close to finished bulking up. Neck collars, especially good quality leather ones, are expensive by themselves, let alone adding the cost of harness with it.
We usually get our leather neck-collar harnesses for our big critters from an Amish harness maker up in Kansas. That stuff looks incredible and lasts forever but it costs about $2500 or more for 2 full sets of standard draft size harness (enough for one team of 2 horses), without the collars. Each collar is an additional $85-$125, depending on size.