I think you need to check the specifications of Zeroshell again.
There is currently no web cache module in Zeroshell. Fulvio has it on the to-do list.
Load balancing and fail-over are pretty new to this distro, as well. From what I have read, fail-over is going to be somewhat problematic, anyways, unless you manually change the gateway configuration when a problem occurs.
Load balancing is a round-robin effect. It won’t help if people are connecting to specific IPs in the LAN sets – it is more of an outbound thing in the traditional sense. In other words, load-balancing is helpful to the users inside the network, not those connecting from outsite the network.
Bridging does not ensure low latency. Technically, neither does clear-channel or any other wireless solution – it is the nature of the connection that you will incure a latency penalty. If you add ANY processing, be it bridge, router, virus scanner, web proxy to a connection, you can expect an increase in latency. If adding a device to your network lowers your latency it is due to the fact that the device / connection you replaced was overloaded, and thus you are relieving a problem in the network.
If you have 2000 users, you need to consider some muscle in your hardware. This isn’t a ‘one box’ solution. If for no other reason, it should not be a one-box solution due to the fact that you are just putting everything to one point of failure again, which supposedly is the reason that you have 2 – 4 connections per site in the first place.
Expect at the enterprise level to need to pay some money. It will be in something – maybe it is in your paycheck. If it is in your paycheck, I suggest reading a lot more on networking in general to make this solution work. While Zeroshell may be useful and even a decent solution for you, the understanding to do what you are asking is not going to be easily answered in a post on some forum. I would also be of the belief that, even if I could completely answer your question, I probably would not be willing to utilize that much time and effort for no compensation – and neither would any of the other people who probably qualify for a CSIE.
Fulvio is a blessing to the community in that he takes time to make wonderful projects like this available to us.