You have two or more requests for help, I think.
1. It sounds like you want a Linux host to act like a windows domain server. In Linux, Samba provides some imperfect functionality of the Windows domain software stack. If you configure Samba as a desktop authenticator *only*, this might work. But it depends on the details of what you are trying to accomplish.
2. You want the new PC's and the new domain to co-exist with the current domain and get DNS and DHCP service from your windows DC? This shouldn't be an issue. Don't install a DHCP server on the Linux box. When joining the PC's to the Samba-provided domain, you will probably need to manually specify the domain controller. Oh, and add an entry for the new Samba server in your AD DNS.
FYI: Microsoft and "reduce costs" are permanently conflicting concepts like "fire and water."
Last edited by mpapet; 05-26-2010 at 04:44 PM.