Hello and welcome!
-Yes, it is best. You can run a VirtualBox MD (see the wiki page http://wiki.linuxmce.com/index.php/VirtualBox_MD
), but it's not a solid solution, performance is much better if you netboot. My suggestion would be to stop using windows 7
Yes you can access the files on the core from a "normal" windows or ubuntu desktop.
-A standalone core, in theory, doesn't need ANY graphics card as it can be accessed from other machines. Although if like me the thought of that makes you nervous, then have a monitor attached to your core. If you think you are ever going to use it as a hybrid, get nVidia. Else, you may be alright with any old thing. I have an ATI card in my core which runs UI1 (although I do have to install the drive manually), which does me. But that said, for the price of a graphics cards, why not just get an nVidia one.
-Use the core as your router - that is recommended. This gives you a "linuxy" firewall there. Plug your wireless router into the internal network, and set up your wireless in the normal way, using WPA, MAC filter, etc. It's important that you realise that this is merely being used a Wireless Access Point, and not as a router as such (check out my setup page, this is what I have done).
-I had nothing but issues with Vonage; my understanding is that they don't "give out" the information required to integrate the line into LMCE, somebody please correct me if I am wrong. FYI I am only talking about the LMCE integration side of things here - apart from not being able to do that, Vonage were exemplary in my opinion (you can still USE Vonage in the normal way, it just won't be integrated). I am now with Sipgate, and have registered the line successfully with LMCE, but haven't been able to dial out/in for some reason. I need to investigate this.
-Yes, older hardware can actually be better in some instances. Feel free to post your hardware specs for us to take a closer look.
I wish you the best of luck. Check out the various user setups on the wiki as well, as they will give you a good idea of how to approach things.