Help with DD WRT!

[TheLeighton]

Use the diagram below to help yourself reference what I'm speaking about when I mention it.
[SPOILER][/SPOILER]
Extra info:

• The "Server" computer has an openvpn server on it(windows7) that I can connect to using the openvpn client, it does not require a password or username to connect.
• The "Server" computer also will be hosting all of the files I would like to be able to access from all locations of my business.
• All computers are running windows 7
• I mainly only need filesharing.

So what I want to do is buy a DD WRT router for each of the 3 different locations that I would like networked together, and connect them to the "Server" computer's OpenVPN server.

I'm basically asking if this is possible, would I be able to file share and everything to and from the "Server" computer.

Here are some questions I have:

1. What router should I buy for each building?
2. Would this one work? http://www.amazon.com/DD-WRT-Router-Lin ... wrt+router
3. Will I be able to file share between computers on the network?
4. Will this bring them all together?
5. Am I doing this right?