Definately go for the SBS suite fella! I have it with now 14 clients and it is much less hassle for SBS than any other AV.
MAKE SURE you completly remove Norton or McAfee crap BEFORE you try connecting the clients to SBS!!! Very important and will save you hours and hours of headaches!
ALSO - Go straight for the ADNM and net-client setup for Avast. I wasted time trying out the Pro version on client PC's.
I run SBS2003 Premium R2 and it seems to like Avast AV software.
Avast is not perfect but what is? It is great value for money.
The Avast sales team are good. Just tell them which version of SBS you have e.g. standard or premium and number of clients (14) and they will tell you exactly what downloads and how much.
No - I'm not on commission.
Cheers