There are several ways to do what you're looking, and a lot of it depends on cost. One way is to purchase mac os X server, which comes with built in VPN server software.
Another, more affordable, solution, is to purchase a hardware VPN/Firewall router. This allows you to setup this box to handle all of your VPN requests, and it scans everything with its firewall.
These boxes don't cost an incredible amount, I think they start at around $100. If you do decide to look for a box, make sure it can be the VPN server. A lot of boxes that say VPN on it, only mean they pass through VPN requests.
Hope this helps.