This is not a very smart solution, as SME Server is not designed to be run in such a setup.
If you only have one NIC you are in a catch-22: In server-only mode, which is default when having one NIC you are exposing far too much services which should not be exposed, making your server very insecure. In server-gateway mode the WAN side, exposed to the external net exposes a minimum of services, but you need two NICs for that.