There are many advantages and disadvantages to purchasing SBS.
SBS is a bundle that gives you not only the Server OS but also Exchange Server, SharePoint, remote web workplace, a lot of wizards to make management easier.
The premium version includes ISA and SQL.
All of the above at a fraction of the cost for the products separately.
The restriction is that SBS is limited to 75 users maximum.
If you are going to be using this for a small network under 75 users, it is the most economical. You must be prepared to learn a little about the way SBS is different. For example you need to use the SBS wizards to do a lot of tasks such as joining PCs to a domain or things get a little wonky.
I hope you find this post helpful.
Regards,
Mark