If someone or something malicious gets a shell account on my systems, then it at least stops them doing anything system-wide. And yes, if a script is going to request admin rights to do something, it'll stop right at the sudo prompt. Passwordless, it could do stuff without you even being aware of it.
Whether or not this is a line of defence at all is open to debate.
Heavy computation rack is in an unheated conservatory with a window cracked open. Keeps the HDD temperatures around 30 degrees. Temperature monitoring from my PDU shows a 3'C rise from the inlet to the exhaust side of the rack. This stuff is mostly powered off when not in use. In summer, it can get to 35'C in that room so I shut everything down at that point.
24/7 rack is in my lounge and vents the heat into the room (helps a little bit with heating costs). Top of the rack is about 37'C but I've seen it around 45'C with all my hypervisors doing stuff. Nothing complains. As long as the intake air is within the manufacturer's stated range, it's fine.
Might want to consider redirecting the heat into the house rather than venting it outside.
DIY - No Regrets.
I built my NAS out of spare parts originally and then it evolved into needing dedicated purchases. I like having full control of the OS and everything on it - it helps me understand what daemons are doing what. It does a lot more than file sharing.
The likes of QNAP and Synology may make a more polished product with an easy UI, as well as offering support, but as far as I care, I am support, so I like to fix problems myself.
If you're ping-ponging between the two options, from your post it reads like cost is the biggest problem you face. But as you say, storage is a critical part of the infrastructure and sometimes you do have to spend money on it if you want it to be reliable. I just upgraded my main NAS with a larger chassis and motherboard (from an ITX) so I can expand it further. It cost me a sizeable amount of money that might have bought me a low-end ready-made, but this is far more flexible.
Yes, this should work fine. SAS does not care what path the signal takes - it doesn't differentiate between internal and external. You can run internal over external cables without issue. I've done similar by turning my old NAS chassis into a DAS, and connecting it to her internal ports of the HBA. And you can connect SAS or SATA drives to the DAS (system 1).
Why not just use ThinOS? IIRC it supports the NX protocol that NoMachine uses. If not, then you have Wyse's respin of Ubuntu called ThinLinux, which you should be able to install.
You can download the OS image and flash utility from Dell's website.
Can you tell if the backplane supports SGPIO? If it and your RAID card do, and things are wired up correctly, you can tell your RAID card to Identify the drive and it'll blink the LEDs. I just configured a Supermicro chassis to do this with an Adaptec controller.
Several locations.
And after all this, I do... Nothing with this data.
Can recommend my APC SMT1500I. Original batteries lasted 9.5 years. Accidentally plugged a fan heater into it once and it survived. Most reliable device in my rack.