Storage Help (NAS+Media Streaming+RAID+Automatic Backup)

Well said!

In terms of HW, after all the great information provided, I think that the best setup for my current needs would be:

*DATA NAS*
Enclosure: Netgear ReadyNAS 516
HDDs: 4x4Tb ST4000VN000 (Gross volume = 16Tb >>> Net volume (RAID6) = 8Tb)
Connection: Directly connected to my Netgear R7000 router via physical GBit/Ethernet port

*BACKUP NAS*
Enclosure: Netgear ReadyNAS 516 or Netgear ReadyNAS 314 (still deciding based on $$$... boy I thought this ball game would be more affordable... :) )
HDDs: 4x4Tb ST4000VN000 (Gross volume = 16Tb >>> Net volume (JBOD) = ~16Tb ...little less indeed)
Connection: ???
Obs: I don't plan to keep my BACKUP NAS on and connected full time. I'm planning to run and connect this box only when I wish to backup data on DATA NAS, depending on my needs.

Questions:
1. Which would be the best way to connect my BACKUP NAS?
a) By also connecting it to the router via GBit Ethernet port (DATA NAS----GBit/Eth-----Router-----GBit/Eth-----BACKUP NAS)
b) Wirelessly (is it possible? Latency would be probably an issue...)
c) Directly to the DATA NAS via GBit Ethernet port using crossed cable? (Router------GBit/Eth------DATA NAS-----GBit/Eth X------BACKUP NAS)
d) Directly to the DATA NAS via eSATA port? (Router------GBit/Eth------DATA NAS-----eSATA------BACKUP NAS)
e) None of the above... :(

2. If I choose RAID6 for DATA NAS with 4x4Tb HDDs, in the future, is it possible to install a 5th or 6th HDD on the fly... so that RAID6 automatically rebuilds the array now adding 2 new disks and also without the need to connect a BACKUP NAS to manouevre data on the rebuilding process?

3. How complicated (or easy, depending on your point of view) is it to configure RAID6 (DATA NAS) and JBOD (BACKUP NAS)?

4. If I choose RAID6 for DATA NAS with 4x4TB disks, I would have a total net amount of 8Tb of space. In this case, is it enough to have just 2x4Tb disks (JBOD) on my BACKUP NAS or should I install the exact same physical amount, independently of logical amount (net anount of space)?

5. In terms of I/O, 4Tb on a single HDD sound for me like "too big" for a fast throughtput/performance. Would it be better to use 3Tb or even 2Tb HDDs instead?

6. In addition, how can I map BACKUP NAS (JBOD) to get data from DATA NAS (RAID6). ReadyNAS comes "from factory" with this feature or should I install some software like SyncBack Free?

7. I surely won't be using RAID on my BACKUP NAS. JBOD sound like the way to go, but are there any other reliable options or JBOD is really the best pick?

8. I've read something about Synology (DS1513+ or DS1812+) and QNAP (TS-670). Is Netgear ReadyNAS really the best pick? Would they fit/work/perform nearly the same for my needs or is there any better candidate?

Average Prices - Diskless (Amazon):
Netgear ReadyNAS 314 - 4-Bay - $564,50
Netgear ReadyNAD 516 - 6-Bay - $1,109.99
Synology DS1513+ - 5-Bay - $799,99
Synology DS1812+ - 8-Bay - $1,002.73
QNAP TS-470 - 4-Bay - $1,049.00
QNAP TS-470 PRO - 4-Bay - $1,349.00
QNAP TS-670 - 6-Bay - $1,149.00
QNAP TS-670 PRO - 6-Bay - $1,482.00

Tks in advance.