We are considering purchasing a Mac Mini with Lion Server for our office as we have downsized considerably and the current managed server is a bit too big (and expensive for us!).

We would therefore be looking to use the Mac Mini as a file server and email server. There would be 5 users who at present are on Windows Laptops although we are planning on moving over to Mac Books as the PC's fall over.

I have a couple of questions on the set up of the network.

Does the Mac Mini with Server act as a gateway for users to connect to the internet? I guess it doesn't.
Am I right in thinking that the best way to connect it will be to use for example if ISP is BT - the BT Hub connects to the broadband connection and the Mac Mini is connected via ethernet cable to the BT Hub. How do other machines connect to the Mac Mini? Via WiFi? Or do I still need an ethernet switch?
Does the Mac Mini need to have firewall software installed onto it? Or will the software that is supplied on the ISP router be acceptable?
From what I have read you can still use the Mac Mini as a normal Mac whilst using it as a server - are there any network performance issues related to this?

I have a very basic networking knowledge as you can see so sorry for any daft questions!

I hope you can help.