--------------I appreciate your desire to build your own,
I used to work at MicroCenter Computers in St. Louis park Mn. and have assisted many PC enthusiast in picking out components. Although....this was the year 2000-2001....everything has changed.....
Sometimes you just can't talk a guy out of a build it yourself mode. and thats fine. I've built a few PC's.
These guys commenting on your situation, are very knowledgeable on the forum.
I took Fendermans advice after my Frankenplex didn't perform stably. removed an i-3 from a Optiplex 7020 inserted an i7 3770S, and had thermal issues.
Found a Dell Precision heat sink that fit into the case and got it down to 72-75C.
but may have suffered from thermal throttling.
Couple that with an SMR drive doing its magic tricks of overflowing cache buffers, and you get a frozen PC.
I got the Hp Elitedesk G4-800 i5-8500.
It's running at a Condo with 19 Cams, and a DVR feed of 9 cams, and a WD 8TB purple, and a WD 5TB white label ( likely a shucked drive from a WD passport- bought on ebay)
It's quiet, energy efficient and the " Tool-less" design makes working inside a pleasing experience.
Also on my list of recommenations for reasons of expandability is the Dell Precision 3630 Tower, touting 3-3.5" bays, Nvidia RX3070 support or full size graphics cards, with the Power supply rated to suppport them.
I do like building my own systems when I think it makes sense. I used to build my own motorbikes for the same reasons - I enjoyed building them, I could make them to fit my needs precisely, and I could trust that everything was put together right. And if I did have a problem, chances are I knew exactly where to look first. ;-)
In this case I have some specific size requirements that put me into the Micro-ATX form factor. I also know that I may have to use this system for another application someday, so I want to make sure there's room for a modest GPU and slots free for future use.
Thanks again for the SMR/CMR lesson. That doesn't seem to be listed in the specs. Where do I go to find out which drives are which?