Parapunter,
There are two obvious approaches - one is to simply replace the Belkin device with a new one of similar funtionality but that also functions as a Wireless Access Point.
So whatever the Belkin does for you in connecting to the internet, firewalling, serving DHCP etc. etc. the new device would do, but it would also act as a WAP.
The second approach would be to add a WAP-only device to the network - this would connect via RJ45 to a port on the existing Belkin unit. Note that this might have to be a cross-over cable, depending on whether the WAP has an uplink port, or the Belkin has a MDI / MDI X port.
The former is probably slightly more expensive, but less of a headache to configure correctly.
You would probably also need to configure the Belkin and the WAP to have non-overlapping DHCP address ranges - assuming you are using DHCP, rather than static IP addresses. Of course it is possible that the WAP you get would simply forward the DHCP request to the Belkin.
Standard WEP or WPA and WNIC MAC address filtering should provide sufficient security - you may also be able to turn down the power of the WiFi network, being in a small space!
If your PCs are at all elderly, watch out for the version of PCI the WiFi cards are vs. the version of PCI slots in your PCs - especially if going for the newer 108 Mbps cards. I posted a warning about this a couple of weeks ago.
You are probably wise in going for kit all from one manfr., but I wouldn't constrain yourself to buying a "kit". Probably easier to find the WAP you want and then choose WNICs from the same supplier.
Also worth pointing out that if your PCs support USB 2.0 you can get USB WNICs - never used them myself, so can't comment on reliability. I wouldn't use them with USB 1.0, though.
HTH
SD