As has been said, if your router is compatible, you can get away with using something like a dyndns.org dynamic domain name.
Otherwise register your own domain.
I did the latter through
www.discountdomainsuk.com, then get a dynamic dns service and update your name server records to be those of your ddns service (so you'll need to register your domain with a company that gives you full control over it).
I set up my active directory to be the same as my registered domain. This helps with accessing anything internally and externally which uses the same certificate etc.
Just remember, if you run any external services such as websites, you'll need to add them to your SBS DNS server.
I use easydns.com for my ddns service as anything .domain.co.uk gets directed to my current IP which means I can run several services.
I can also add static entries such as a website hosted by an external company.
With them you need a ddns client, this checks your current IP and updates your ddns service provider.
You'll need a smart host service too, I use authsmtp. Set your exchange server to forward emails to the smart host, and it will be sent on your behalf without the recipient knowing. This gets around the problem of recipient mail servers detecting that you're sending from a home address and blocking the mail incase it's spam. It also allows me to get around the port 25 outbound block that my ISP sensibly has in place as authsmtp can handle connections on another port.