You could setup OpenVPN on a machine that has access to the internal network and the Internet (so your sales people can connect to it).
This would create a secure tunneled connection into your backend, and its certificate based so its very secure.
This would be free... (except if you didn't have a machine you could use to act as the VPN server, then it'll be cheap and you just get a used Atom on eBay for much less than $500)
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Those EDGElite routers look awesome. I'm already familiar with Vyatta. I'm thinking of ordering one just to play with it. Guess I will hit up the community forums soon and make sure it supports what I might use it for. :-)
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Save your budget. Get a desktop machine and install pfSense, best thing in the opensource. Yes include netbios support. Configure the openvpn server using the web interface, then install the package OpenVPN client export, go to its section after installation and get the client package with the conf already built in it, then just run the downloaded package on the client computer, and you are set.
Another option that hasn't been discussed is picking up a used Juniper SSG5 Firewall. We picked up several of these off eBay for ~$125 each, and although they aren't the current model, they've been nothing but reliable and solid in our environment, and are serving their purpose well. The VPN is easy to setup, and you get the bonus of having a hardware firewall.
Another option is to use SoftEther VPN, which is free, and you can find it at http://www.softether.org/ We have a couple of clients using this, and they have always had good things to say about it. They simply run it, for example, on their domain controller (no need to devote a machine to it), and connect via the VPN client, and away they go.
With this, and the rest of the answers, you should have a number of directions to go that are all valid.
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