Hi Suhail,
Yes you are right, in an Active Directory Domain PEAP can be implemented transparent to the user, so that the NTLM authentication is done by NAC. Nevertheless the NAC Appliance need a server certificate so that the 802.1X supplicant trusts the NAC as RADIUS Server. But be carful in an multi-forrest, in this case it can be easier to use the NAC as RADIUS-Proxy to an Microsoft NPS Server as PEAP needs an AD membership. But if onlye on Domain I would use directly NAC.
With EAP-TLS from my point of view this is a lot easier as you don't need the AD integration with NTLM. NAC needs a trusted server certificate of the Domain CA and the beloning CA certficiates of the client certs. That's all. The fact that the client uses a trusted client certificate is enough. If you want to check if the user (or mostly better - machine) has an active AD member account you can do a simple group membership check based on an LDAP-Bind. (memberOf CN=MyGroup,DC=myDomain,DC=local).
That's the way I implement most of my NAC projects. EAP-TLS combind with LDAP-Bind memberOf check.
The NICs are normally used seperatly. E.g. you can user 1 for Mangement and RADIUS and another for handling Guest Portal traffic. It is not intended to create a LAG.
What kind of assessment are you gonna to do? Agent-Based Assessment or Agentless? If agent-bases I would not worry of 1G. With agentless you could use a second NIC. In general I would recomend to use at least 2 NAC Appliances (hardware or virtual) to have the redundancy. Virtualization is no redundancy if you think about Updates ect. .
Hope that helps. If you have any further question don't hesitate to ask.
Regards
Michael