I'm currently using a Dell PowerEdge R710 with 72gb ram, 250gb ssd and 5x 4tb. Using Windows Server 2012 r2 and storage spaces. Plex is a VM using CentOS on 4vCPU and 4gb ram. I can always allocate more if needed. Having a gigabit fiber internet with AT&T helps.
I tried using windows 2012 r2 for plex but it would sometimes crash for no good reason. I had to learn linux which I hated at first because I am windows and gui all the way :) Now that is it in linux and 100% stable. No way I am going back.