need help re. office network install

I'm looking for a little guidance/explanation on the ins and outs of
basic office networking. I know enough to understand what I'm talking
about but lack the experience to make a confident choice in this
situation...

I'm currently working as the IT/developer for a small (20-person) firm
and their network is a mess, the result of years of neglect. They're
still using static IP, they have a gateway server w/ no special
firewall rules on it, they have a large DMZ that serves no purpose
(managed by the gateway) and are fronted by a Cisco router they can't
get access to (nobody has the password; I presume that this is
performing NAT).

I've called in a few network install technicians to get some quotes and
they keep suggesting rather expensive ($2000-3000) appliance devices -
clearly the ones they have the most personal experience with.

What I'm starting to wonder is why can't I simply do this myself? Why
not just buy one or two switches and a Linksys router of some kind and
set it all up myself?

As I mentioned, we have about 20-25 desktop machines that I want to
configure via DHCP services, 4-5 printers (which will require static IP
addresses), a file server (no outside access required) and a print
server (for the 4-5 printers). Web server and e-mail are both hosted by
an external service.

I'm looking to implement a protective firewall, DHCP services and
possibly VPN access in the not-too-distant future. If I ever intend to
bring web and e-mail back in-house then I'll need port forwarding for
that, as well.

Considering that we already own one 3Com Superstack 3 24-port switch
(and a slew of Baystack 255 hubs that I want to unload), I was
considering just getting one or two more Superstacks and a Linksys.

Am I being foolish in thinking that something as small as a little blue
Linksys like what I have at home can be relied on in a small/medium
office environment? Is this too taxing an application for one of those?
And what about the difference between a BEFSR11 (the single-port blue
model) something like the RV016 or RV082? Considering that I've already
got switches then is there any advantage to having ports built into the
device?

Any input, experience or suggestions would be greatly appreciated!