Recommended hardware for 100mbps connection
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As tittle says, 2 ethernet ports minimum, 10/100, low power (as low as possible), stable
Prrobably 3-5 users maximum, might run Ovpn squid, squidguardEDIT : Mistype
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An SG-2220 would be perfect for that.
If you are going to run squid I would advise getting the additional storage.