Generally dvi and analogue out puts are present in the card, since dvi and hdmi are both digital signals it may be the card is automatically giving preference to one source, you will probably have to fiddle around with the external gfx control panel to enable both digital signals
As far as...
You can easily create 3 displays using any ddr3 gfx card and one you can get from the onboard display, for the rest you need to insert another gfx card and theoretically it should give you three more displays ( have not tried this though ) alternatively if ur monitors support it you can use...
You can only upgrade the pc with the os to win10 till 29/7 and you can also directly install win 10 using 7/8 key however you can't activate over vm because your key is tied to your mobo and you will end up activating all your licenses on same mobo
My take is that you don't actually require a nas , a media server would do just as well.
You can just do network share of your media drives and access them from whichever devices you have on the wifi or you can attach a large hdd containing all your media directly to your router if it has USB...