Ok, well just a few suggestions.
Since you are running an SBS server, the pricing is much better if you go with the SBS suite. Since the Suite already includes a license for ADNM, why not use it, right? Sure beats installing and managing clients one by one when you could control them all from one place!
As for the filters and blacklists, I hear ya. I was used to setting things like that up myself, with other A/V programs. Then I learned that Avast doesn't really need them! If there is a problem with a website or an IP address, it's because it's a bad site, or it has been hacked. You pretty much don't need to whitelist or blacklist ANY site. It just works that well.
If you want to make sure that you have everything that you need, that you don't have the tools for, make sure that you get some extra malware protection. Avast pretty much detects everything, but there are some times when it doesn't.
http://www.malwarebytes.com and
http://www.superantispyware.com are good products to use if Avast is out-smarted.
As for the larger clients that you have, if they're used to symantec and you switch to avast, they'll definitely notice a difference. The speed increase is incredible. I used to have Mcafee on a few of my sites and I switched to avast. It's so much better!
Good luck and welcome to the forums.