From the design of the network we can only guess why the computer guy was fired. Doesn't do networks?
Having 'all in one' equipment is looking for trouble. Something breaks, you go down. At least with multiple boxes, only part of your network goes down.
Go find a linux guy and have them build you a DHCP server. Better yet, make it a full blown router/firewall with DHCP installed, along with IPTables. Properly setup you can't be broken into from the outside. Depending on your Internet connectivity, a pent 200 with 128Meg will run circles around any of the 'off the shelve' router/firewall combo's AND will wait patiently for data from your ISP.
Easy to expand. Years ago, one of my customers needed help. I setup a
4 user network. Since then I have added stuff: I am currently maintaining a 55 computer network, DSL ( 768/768 ) along with a Windoz server box ( couldn't talk'm out of that ). The Windoz box is a file server on an old Proliant 5000 and fills the local network 100Mbs pipe. I split the local network into to ( 192.168.100.0 and 192.168.200.0 of which the linux router/firewall handles fine. Has 4 16 port switches ( 2 on each network ) to help in the transfer rates of the client/server sharing. The DSL circuit hits 75% utilization multiple times a day, but I can't get'm to go faster as they don't want to spend more money and can live with the current limitations. Time will change that.Oh, as a side note, that linux box should be fairly inexpensive. Find one of your 'old' computers ( pent -> 66Mhz bus class at least ) and use it. I don't know were you are, but getting a linux guy into the action shouldn't be to bad. Heck, you probably know somebody ( or do it yourself ) that can help you out for a few hours of consultation. If you wish to go this way and can't find anybody, give me a yell as I will build ya one and ship it to ya.