Log in to the virtual box with your normal account and use
su to get an admin account.
/etc/network/interfaces in an editor.
To set up a static IP use
iface eth0 inet static
Sadly I am not sure what you mean with DHCP IP for the NAT adapter
To setting up a simply DHCP IP for eth1 use add
iface eth1 inet dhcp
to the file you just edited.
If you want this line to connect to the outside world with and actually do NATting then you will need to enable IP forwarding, load the MASQ module and set up some routing. If that is you goal then please clarify your post.