New NAS Build

[PenguinMaster]

I am thinking about building a new NAS. This is my first real NAS, so correct me if i'm wrong about certain things. I had made previous posts on the topic, but they are now outdated since I wasn't ready to purchase at the time. I am looking for a storage server mainly for backups and a plex server. I was thinking I could combine the two into something like a Dell PowerEdge r720XD but if someone thinks they should be separate, let me know. I have a mis-mosh of random drives so I think I need to use UNRaid as the main OS and then I could use CentOS for the Storage Server and Ubuntu for the Plex Server. For specs I was thinking 12 cores and 64GB+ of RAM. I think I need to allocate most of the resources to the Plex Server since there could be multiple people streaming off it at once, where as the Storage Server probably wont have more than 1 user accessing it at a time. I would like decent transfer speeds however. I was thinking I would need to add in an SSD for a L2 cache, but I saw people recommending to not use a L2 cache. 

 

These are the drives I have:

2x 1TB 7200rpm HDD

1x 320GB 5400rpm HDD

1x 1TB 5400rpm HDD

1x 750GB 7200rpm HDD

1x 500GB 7200rpm HDD

 

These are all Desktop-Based HDD's and as they die I will replace them with NAS equivalent HDD's. My budget is $350, but there is a little wiggle room if necessary. The UNRaid License is not included in that cost, but if someone had a Free-Alternative that would be appreciated. The only thing that I can think of for other storage options are HP Proliants, but I don't know much about them. Is the Dell PowerEdge r720XD too old? All suggestions are appreciated too.