For that many users I highly recommend you don't go with a "off the shelf" router. You'll run into performance and connection limitations. I also can't tell you how many Linksys-type routers I've replaced that have died for no reason.
With that said, I've had lots of success with the Safe@Office 500 series (available in both wired-only and wireless models)
http://www.safeatoffice.com/landing/
They have a licensing scheme, but it's well worth the money. It's essentially an enterprise grade router on the cheap. It has a nice web interface (much like your basic Linksys, Netgear, etc) so the learning curve won't be steep on it.
They also have a yearly subscription fee (about $99 IIRC.) Again, it's well worth it. It automagically updates its own firmware plus gets new virus definitions.
It's a VPN endpoint (letting you do user->router VPN and router->router VPN) and comes with 5 licenses (more can be purchased.)
There's also a upgrade (called the Power Pack) that will let you create a DMZ along with some other features.
I swear I don't work for them I do, however, setup networks for small businesses, so this is right up my alley. A competitive product (with similar features and pricing) is the SonicWall TZ170.