OS X Server safely facing WAN and LAN?
Nothing much will change in regards to WAN exposure, as long as you're not configuring the server to proxy or route any traffic. Services intended for the LAN only should be configured to only run on the appropriate interface, regardless of whether there's a firewall in place.

There is an increased risk if one of the externally facing services is compromised, potentially allowing access to resources on the LAN. If you want to be really careful you can set up the firewall on the Mac to not allow traffic out the LAN interface unless it originated on the Mac...