What internet "pipe" do you have? Is there a "minimum" speed you want to give to users, or is it "best effort"?
How are you going to control abuse? Different type, like sending bulk mail, flooding with IGMP packets the network etc....
Before you buy / think about equipment, first think about these things. In certain countries the users are "always" nasty, and trying to break the network or something. In other countries, there are brave internet users, which do not do anything more than browsing (official) websites, and do Facebook stuff....
That means you need to control this.
Also, in certain countries you will be considered an ISP, which means, that you need to log almost EVERYTHING the user does, for 1,5 years! Do you have enough storage and software to do this?
Currently I have a 4 mbps internet connection, I am planning to add another similar line and merge both of them. And I am also planning to limit users connection at 512 kbps.
For controlling abuse, I will try to set the best security configuration, at this stage of my knowledge I believe it's all about good configuration. Do you mean that certain vendor hardware is better in security than others? please clarify!
Thank you