1) is this nitris Dx that you are talking about? Capture from tape?Depends what type you're looking for. The Nitris Box I use isn't made anymore as Avid moved away from the hardware game and basically just gave their business to Black Magic. A comparable product to the avid hardware is the UltraStudio line which conveniently does work via Thunderbolt so no need for external PCI. They range from around $140 (For just HDMI and SDI Out) to $2900 for a full studio version that has everything you could ever possibly need for IO.
Honestly though, unless you need a client monitor, you could go without hardware. If you need a full screen monitor even the lower to mid-end Black magic would do the trick, unless you need 4K. I believe that you only get 4K playback support from the top two tiers which will set you back $1500+.
We use the Nitris box because we still print / capture from tape and colour on a broadcast spec monitor. However if you don't need those capabilities than software only or a very simple client monitor box would do the trick for you.
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As for your questions regarding Software choice. As mentioned, Avid is king in a shared storage environment. Unity, Facilis, ISIS (terrible name Avid), etc all work amazing with Avid but they are costly and not at all necessary if you are working on projects on your own. If that's the case, most people do find Premier does very well. It's about the only main NLE I don't have experience with so I can't say for certain but I know a former colleague used avid for %+ years and switched seamlessly to Premiere and loves it. Barely any transcoding, strong features, and a solid price considering you get the rest of the CC Suite as well.
Definitely try them both out(although obviously stagger the trials to get the most out of both) and see which one you personally like more.
2) avid is king of shared storage? Sorry I'm trying to understand what avid all about.