Why do you want to spend money on an "appliance box" which will just be an Atom Server, running Debian 6 and Squid?
Just get a cheap server, run Debian 6 on it, and install Squid.
Seriously. Appliance boxes aren't all they're cracked up to be, generally. Sure, there's "support", which is expensive. There's also the risk that they won't be quite as pro-active as they could be on updates, and security patches, and your new-fangled appliance box, actually becomes a weak point in your network. You won't know about this, because it's a hands-off black box.
Screw the appliances. Get a server. Run Squid on it. If you need support, use the community, or hire a contract sysadmin.