Amahi will work behind VirtualBox NAT quite happily, but at the time of writing, VirtualBox port-forwarding doesn't play nicely with Amahi's network config. (Or at least it didn't for me.) NAT also won't permit two virtual machines communicating (e.g. for testing PXE-booting).
If you want Amahi within the VM to serve other machines on your LAN, select bridged mode instead of NAT initially and don't alter the network config. (The basic install process will ''just work''.)If you are running on a Linux host, you can go into the "Advanced" settings and set the "Adapter Type" to "Paravirtualized Network (virtio-net)". For more information as to why you might want to do this, check read [http://wiki.libvirt.org/page/Virtio here].