How to connect two small offices over internet

[bjur]

I have a small Structural Engineering company. I would like my employees to be able to work from home and connect to my office NAS (network attached storage). Is there any way to do this securely? My office uses macs for just about everything, however, we still need to use windows (running inside VMWare) to run some analysis software. Currently I store all my data on the server (NAS) and I can access it from both my Mac and the Windows portions of my computer at the same time. It works great, however, how do I set this up when the employees work from home. Do I need to set up a VPN? Do I need a static IP address or can I get by with a dynamic one and do some kind of port forwarding to the NAS. Am I way off and need to buy a fancy server instead. All we need is something to store files and be able to access that data from either home or the office. Again, we only use macs, but the PC portion of the computer needs to see the server as well. Thanks in advance.