Back from building and installing the setup... Here are the results. Hopefully you can give me some more advice what else I need to do.
First off, I started to marry the
hardware:
Everything is connected in place: PSU, 8x 2TB WD RE4 drives, 2x SATA-DOMs (only one shown in the pic) Frontpanel connectors and 3 of the 5 fans (waiting for the power cord extensions)
After that, I looked at the backplane of the chassis. Because the fans are connected directly to the mainboard, I removed the jumpers for the backplane fan-connectors to not get any red LED's and connected all 4-pin power on the backplane.
As this is my first build of this kind:
Are there any other thing I should/could connect? (Backplane to MB or something like that)
Software:
Installed FreeNAS from a physical connected USB stick, remotely controlled via IPMI. Nothing special to say here. Mirrored the boot-pool to the second SATA-DOM afterwards in the webGUI.
Config:
I think the main thing to mention are the system fans. Currently I run them on "Full Speed" because the power cord extensions are currently not available for one "backplane" fan and one on the back of the chassis. A bit loud, but HDDs are all at 34-37 °C, while running badblocks.
"burn in":
As stated before, this is my first "big" build. (Yeah, some of you guys think this setup is tiny and cute ;))
Followed the
How-To by qwertymodo.
badblocks #1:
First run first round I got about 50k of errors after 7h on 6 drives. WTF?

Yes, you are right... my fault... Tested some things and created a pool with 3 mirrored vdevs and 2 spares. These errors had to be thrown.
badblocks #2:
First run, first round took about 7h to get to 40% of writing. WTF?

Yes, you are right again... It was past 3 o'clock at night and I unfortunately used "-ns" instead of "-ws".
badblocks #3:
Currently running at 0x55 reading and comparing at 80%. No errors so far.
These are my experiences so far. Hope to get the missing power cord extensions for the system fans as soon as possible and will report back on the HDD-temp.
Your turn:
If you have any recommendations, suggestions or questions, please let me know. :)
Just for information:
Have a second X10SRL-F with a E5-1620v3 as ESXi 6 running. Fully compatible and running amazingly fast.