Regarding AD/LDAP/Kerberos, I gather it's possible to run an AD DC as part of Samba now. I haven't tried it out yet, but it looks promising.
I recently switched all clients to Linux and plan to abandon this approach as I was after proper user/authorization management between my Windows 7 clients and a Linux based NAS. With Linux the integration via ssh/sshfs is much easier and fits better to the Linux authorization model.
As a comparison: Setting up AD properly on all parts did cost me at least several weekends. Setting up sshfs only few hours. And the latter is much more responsive...