Simply use whatever virtualisation software you want (VMware Fusion, Parallels or VirtualBox) and create a vm that uses the built-in network card of the Mac mini and tell it to use bridge mode (no NAT as this would cause some problems). There is only one downside: when you reboot the machine you need to restart the virtualisation app and restart or resume the Windows vm. VirtualBox has the option to use headless mode for these kind of situations.
Using a usb network card is not really recommended as these cards have some problems. Having 2 network cards is most likely to be an overkill. The only usable thing would be load balancing (which 99,99% of the small offices do not need) and minimising the very small risk of a broken network card (the mini has wifi so that could be used as well, although network cards do not tend to break often). I consider it to be a bit of overreacting, it seems to be like dressing up as the Michelin man when travelling to work in your car because "something" might happen.
In this case I'd check on the memory and I/O usage because those are the biggest problems when using virtualisation. If the vm is using a lot of memory or I/O this means it will slow down the Mac mini considerably which in turn can cause all sorts of problems (files from network shares open very slowly, email is slow, it does not validate your credentials when logging in and thus does not allow you to log in, etc.). You need to investigate what amount of memory Windows needs and what amount you can actually use for the vm. It might also be a better option to run the vm from a different hdd than OS X and your data. You can test these things by downloading the trial version of Parallels (or whatever you want to use) and test it on your workstation. It will give you an idea of what to expect.
