You are almost done with your job...
To configure DHCP there's the wizard (when you add the DHCP role), doesn't matter the external interface, when asked, select the internal interface (private, LAN). Anyway if you want a step-by-step...
Regarding the sharing... actually you are not sharing the connection, or at least you don't have to... you can easily ROUTE the connection from NIC 1 to 2.
Once again to set up ROUNTING in server 2008 there's a wizard which is really easy (your scenario is not complicated), but if you need a step-by-step...