At my place of employment, we have about 10,000 computers and had a setup very similar to yours. Let me start by answering your questions right off the bat.
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No, it will not cause any conflicts if setup correctly. For awhile we had WDS and FOG running at the same time without any problems. We had a WDS.iso that we created that would point to the WDS server and image our Windows 7 machines that way. All we did was add that iso to the FOG boot menu. Simple :). We had our network guys setup FOG on the DHCP server (prots 66 and 67) and never had a problem.
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The security risks are all dependent on what you choose to run. Apache itself is not a concern. You won’t have FOG sitting in the DMZ, so your employment’s firewall should block almost everything going out anyways. The concern is what version of Linux you choose to install FOG on. The more features you install, the more security risks you add. I personally am an Ubuntu guy, so I install FOG on Ubuntu server. It’s all command line, no GUI. This means that the server is fast to boot, very little to crash, and extremely robust. I ended up having to install a GUI so others could administer the server if I was not available, so LXDE core to give an absolute minimum GUI experience for others. But again, less is more when it comes to security.
Now a question I have for you. Especially with the transition to Windows 7, is there a reason that you can’t have your machines on the domain? It seems like it could be very easy to just group all of your machines into a new OU. Group policy is a very powerful tool and you can really tweak how you want your computers to work and behave. With Windows 7, you can have a whole new level of control that didn’t exist in XP, and can install templates (ADM and ADMX’s) to control programs even furhter. Granted you can’t do “frozen” profiles by default, but you can at least limit what gets installed in the first place, which is half the battle.
Unfortunately we have since discontinued FOG as people higher than me decided that we could no longer use it, in favor of another product. I do however, still have a full lab setup at home to do testing on, including WDS and SCCM, so I still get my fair share of FOG in :). I will say, FOG was, and still is BY FAR the fastest and easiest imaging solution that I have used.