If you don't want become a security weenie, I'd suggest that you get something commercial to protect the network. Ideally you'd get a firewall with at least 3 NICs in it, one for an external interface, one for your PCs and one for the transient systems. You can set up firewall rules that allow the catering PCs and the transient PCs to get to the internet, but not to each other.
I have used the Astaro Linux Firewall, it is relatively inexpensive (about $350) and you can set it up to do automatic updates so that you don't have to keep up with the security business. Of course you still have to buy a PC and install the software and configure the firewall rules.
Another option is the Cyberguard line. They are relatively inexpensive (under $5k) and are turnkey systems, so all you have to do is configure the ruleset. They also offer automatic updates over the Internet and come with 3 interfaces.
As far as a network for the transient computers, I'd probably install a small 10/100 switch (maybe 8 ports) and an 802.11b (or 802.11a when it becomes less pricey) and just leave everything wide open.
NAT is, of course, a good idea for both networks. If you have one static IP address, you can define firewall rules for a Webserver and/or Mail server that go to a specific machine on the internal network (preferrably a DMZ with only the externally accessible machine(s) on it).
mgs