firewall specs

Sourcing for a firewall –
for a network with 200 users surfing the web, accessing to web applications and email routing.
In the spec for a firewall, what need to be looked at in order to determine a suitable firewall?
A vendor is recommending Juniper Netscreen, will the SSG520 be an overkill, or will the netscreen 25 or 50 be alright?
Appreciate guidance. thanks.