Hi
1. traps. Not that I'm aware of. Things to watch out for would be port conflicts between WinGate and other windows services, such as IIS (web publishing) and DNS servers. WinGate's web proxy can run on another port no problem though, and it will alert you about port conflicts and the process that is conflicting.
2. if you install the WinGate Network driver, then running other internet apps on the WinGate server will require you to enable the ports in the firewall settings. Some services like FTP servers would require opening a range of ports. Running services on other machines would require port forwarding, but it's not clear to me if WinGate will be the only way from your LAN to your cable modem or not.
3. logging in is optional - in that access to whatever sites can be forced to auth or not depending on your requirements. iOS clients generally have no problem actually authenticating to a web proxy though, but make sure they are running 5.1.1, since earlier versions were buggy in this area.
4. I'd recommend taking a look at the web access control policy video on our youtube channel
http://www.youtube.com/user/qbiknzltd/. You can allow or ban sites per user / IP / time of day. Flow chart policy is still more flexible.
5. Bandwidth rules in WinGate are per IP:port, so don't necessarily map 1:1 to user.
6. WinGate will work fine with only 1 NIC, although it may be easier to do bandwidth control with 2. As for enforcing auth, that's a configuration thing - depends on how you set it up. Since auth is optional. What sort of access are you envisaging here? E.g. if it's web proxy access, you can do that, but WinGate won't enforce auth for file share access etc, since that's done by windows.
7. WinGate licensing is done by concurrent user. I'd recommend a trial, you can see how many licenses were used. If you're also serving hosts on the internet, they will consume a license count each as well.
In summary, I'd recommend a trial. You should be able to ensure it will do everything you want before any purchase decision, and our support desk is open to trial users.
Regards
Adrien