@ally_uk You can do it with a single network card, just be aware there are a few more steps than standard.
As for your question about dhcp server, you must pick Y to install the proper proper programs. There will be a small risk of dhcp confusion while you are installing fog this way so as soon as FOG is installed unplug the fog server from the business network and plug it into your imaging network.
I started writing these instructions for centos 7, then I realized if you are not familiar with linux the instructions may be pretty daunting. Here is a different guide: https://wiki.fogproject.org/wiki/index.php?title=Installation As much as it pains me to say this, probably ubuntu or fedora would be a better choice if you are a gui based user. The link above is the basic installation jumping off point for a number of operating systems.
Once fog is installed and your fog server is in the imaging network. You will need to first change the fog server IP address to one consistent with the imaging address. For this example I’ll use 192.168.101.x/24. So for the fog server we will use 192.168.101.1 for the fog server.
Update the fog server IP address then reboot.
Edit the FOG IP address in
/tftpboot/default.ipxe
/opt/fog/.fogsettings
Edit the FOG IP server and subnet range in for the isc-dhcp server. The file will look similar to this one:
https://wiki.fogproject.org/wiki/index.php?title=BIOS_and_UEFI_Co-Existence#Example_1
Now off to the FOG webgui. You need to update the IP address in 2 places in the FOG configuration page and then for the IP address in the storage node configuration.
I can tell you finding a second interface will save you quite a bit of pain, even a usb ethernet network adapter will work.