Professional Ordinance makes a Carbon 15 .223 pistol with synthetic lower. Click on industry forums on AR15.com home page and scroll down to Professional Ordinance. They have their own forum. I don't have direct link, but you can search for one.
My friend has two of their pistols and loves them. Great post-ban models, removable mags and all. To me, this is the better option than Hesse. Haven't heard many kind reports on Hesse, and I think they've got a rather dubious reputation.
I don't know if pistol kits/parts are that redily avaliable either. Only benefit I see of Hesse over carbon 15 is cheaper parts or parts you already own. Requires at least different buffer and barrel. Carbon 15 is for shure within the law as is to boot.
Anyway, good luck with your project. Let us all know if you get a Hesse built!