Thanks for the reply! Its helpful to have someone else's input.
1. Yeah I figured they weren't much use, but I have them laying around. Probably don't need any of the 2.5" HDD drives right now.
2. HBA is certainly not necessary at this point. My thinking was because of the speeds of the NAS array drives I would rather keep the MOBO sata slots open for SSD cache pool stuff. I have a stack of 2.5 ssds that are not much use otherwise.
3. I'm already living that rack equipment life, but i get a lot of second hand stuff from work, so the $$ hasn't been an issue yet. The rack is in use currently and the locking door is excellent for keeping the little ones out. It is housing a sonnet rackmini mount with one m1 mac mini, 1U PDU, 1U UPS, 1*3U drawer, 1*2U drawer, Silverstone GD09 gaming PC. It's shallow though so cases longer than 21ish inches may become problematic. Of course this will all need a good redo when the server is built.
4. Air flow was definitely concerning with this case. It was kind of a tossup between the one I listed and the RSV-R4000U. Seems that may be the smarter way to go.
5. I was going off spaceinvader's recommendations which are now impossible to find in such small sizes. Any thoughts on the SanDisk 16GB Cruzer Fit USB 2.0 Flash Drive - SDCZ33-016G-G35 ?
I was thinking of it just as throw all this old hardware in a box and use it as needed, but that may just be adding complexity and issues (2.5"HDD and such).
I'm still very tempted to move my gaming setup into a sliger 3U unit. May do that if I need to save space in the future or watercool these next gen hot CPU's. I would like to fully consolidate into one unit, but need to see how the VM works out first.
Thanks again @Lolight