The DNS part will give you problems - Active Directory uses DNS records for all its service-location information, with said extra records being regularly updated/changed/removed by the servers. Trying to replicate all of this on a non-domain-integrated DNS server will, I suspect, lead to pain.
Adding another DNS server for plain (non-AD) DNS resolution is certainly doable (see Evan's answer for more), but since your DC needs to be a DNS server anyway, you're as well sticking with that and sending all the non-local requests to an external resolver, as you suggested. These are listed (and added) in the "Forwarders" section of the DNS server's properties in the DNS Manager (assuming you're on some version of Windows server).
There's no reason that you can't press your pfsense box into service as a router and DHCP server as well, though.