“(a) I want to use more than one internet connection to increase the speed internet in my network”
Trying to understand what you want to do. Just get a higher speed 8 – 10 Mbps Line
You cannot use two Internet line on the same network
What is the amount of users on the network 2 – 200 – 600?
“ (b) I want firewall to protect my network, email filtering, stop downloading etc”
There is all ready a firewall on all Windows OS computers and on the Internet router
Need a lot more information before we can help
How is the network set up, how many, switches is connected, is there a wireless system,
is there more than one network etc.