It's always a good idea to split the services across servers if budget wouldn't be an issues. This will give you a more scaleable environment and one server interruption will not give issues for all the services - i.e. Web one, email one server.
For firewall on Linux, you could try with APF or CSF firewall if you are running cPanel control panel, both works quite well for this. For Windows, the built in Windows firewall works pretty much well. Since you are starting the business, it may worth to keep the cost down and only move forward for hardware firewall when you have enough profits/margin gained.
SmarterMail mail server is a definitely a good mail server to move on.