Yes, you can use zeroshell…
You should calculate the machine that installed with ZS.
Calculate how big the users, calculate the routing, authentication and also encryption.
For example my project, I use Intel Core2Quad, 2 GB memory as Axiomtek NA-821.
I can handle 46 branch with VPN connections, also bonding and authentication for Captive portal.
Anyway, please try another solution to give you more comprehensive view.
Another solution like pfsense, m0n0wall could be another candidate.
The last decision is on your judgement, how easy the administration, how easy to update and upgrade, and the important thing to the customer is no interupt services.
That’s my 5 cents…