Been there, Done that! Numerous times...
There is tons of advice available but for your situation I'd say two things:
1) the Cisco solutions are complete overkill for your network size (not to
mention costly and difficult);
2) the home/consumer models might stumble when all your users get busy at
once;
So, I'd recommend a mid-level "enterprise" class solution.
There certainly are quite a number of them but for your particular solution
I'd recommend you take a look at
www.netopia.com and look at their 3386-ENT
model and if you are planning to do VoIP they have a similar model with
QoS.
You can purchase this at
www.provantage.com and Netopia, now owned by
Motorola, has terrific tech documentation on their site and their tech
support is terrific. And, for a whopping $99 or so they will do everything
they can to talk you through setting this up and if you can plug it into
your cable modem and provide them your public IP they will even configure
it for you for this fee.
No, I am NOT a Netopia salesperson, just a long time services provider to
numerous clients and after installing many Cisco solutions and having
difficulties and not getting much in the way of help from Cisco or
their "partners" I finally had to find something that would work as
expected. And this brand has seemed to do this. At least for me and my
clients. In fact, I just installed a couple of dozen of these used to
connect on-line ATM's on DSL using IPSec VPN's to a bank's edge network.
Works like a charm.
Good luck (but you won't need it!).
Bob