Newb questions about setting up a small office network

This is my first time doing something like this, so it's pretty much a 'learn as I go' process for me. I'm trying to set up a small office (5 PC's) network. I already bought a server and a couple of other things. The equipment that I have as of now:

1. Dell PowerEdge T30 (no OS)
2. Netgear GS105 switch
3. Necessary cat5e cables

I was also thinking about getting a router, but I'm not sure whether I need it (we already have a modem that comes with our internet package)

Another thing is that the server comes with no OS, and I'm not sure what kind of OS what I should get. I want to be able to do the following:

1. Make certain computers only be able to open certain files but not change them (for example quickbooks, excel files) etc.
2. Have another computer in a different building also have access to the server.

So here are my question:

1. Is that something I can do with any OS or do I need to buy Windows Server, and if so, which version?

2. If I need a router, what's the best option?

Thank you in advance