Take a look at IPCop. It installs on older systems and is a IPSec VPN router that allows for dynamic addresses. I have put IPCop at each of my sites and have SME behind it either on the lan as a server only, or as a server gateway, with the external SME nic residing on the DMZ lan from IPCop. IPCop also has built in Port Forwarding and Snort, plus it's web interface is easy and powerful. The IPCop download is under 30mb...but is is only a router..no users, no email, no file or print server, so it is not a replacement for SME.
My life is simpler know that I use SME for stock services.......trying to keep SME up to date while keeping the addon services working was a big headache for me.
good luck,
ryan