One more thing...
For the question....
what is the minimum required bandwidth?......
Based on my observation.... it difficult to recommend customer for higher bandwidth... even if you have 256 kbps bandwidth, issues may occur..(in ur case QOS may solve the problem, but it reduces the updation rate).
Many customer run different network application and preference is given to their application...
If we recommend them to upgrade to higher bandwidth for the reason of AV their primary question will be "Why i have to go for higher bandwidth just for AV when my application works with less bandwidth".
For that reason first we have to explain them what is the updation method and how server and clients are updated...? Why AV Uses high bandwidth...?
Full bandwidth is not utilized all the time..
Utilization goes higher in few scenarios like client switched off for long time, fresh installation, Updation corruptions, Re-installation of AV, etc...