I have one platform running on a home built. It's roughly 7 years old now and running with no issues. I do nothing on it but trade. The second is a Dell that is now about 3 years old and also running with no issues, and other than some financial items, again I use it for nothing else. I wouldn't build one these days for the reasons previously mentioned. Back when I built I was able to find deals on all the parts and saved roughly $200. I helped my son build a gaming computer last year, and despite my telling him he could do better price wise for the performance buying an Alienware or the like he wanted the experience of building. Also it was his money. As it so happens he lucked out and a guy he works with sold him a new top of the line video card he won at a tech show for $250, and he sold the original one he bought for $250. I loved the computers on the site destriero posted, but for the majority of us trading, one of the mainline brands will get you a computer that is great for trading at a much smaller price.
Probably none. Too important for anything but most reliable operation/backup. Besides, proper backup and redundancy are not expensive in most cases. Proper backup is not expensive for any of us. Only reason not to have it is laziness.... but after one big crash/compromise that results in your having to "start over".... you'll have backup from then on. (I recently had a catastrophic failure in my trading rig. All USB functions went out, effectively trashing the mobo. I was back up and running in 15 minutes.)
Even with the Intel Meltdown and Spectre issues that still haven't been solved? There are still old Dell and Hp workstations in production environments right?
They have been doing patches for CPUs that are not "too old". Any business with CPUs too old to be patched should have upgraded. Of course, can't say who did and didn't.
I, too, have a 1U server at home. If you are running a 1U rack server at home you can probably lease computing time for college kids to render art projects on. That's what I'd do but I'm too lazy to sysop these days.