Personally I’m too paranoid about security and sus of Intel to be comfortable with vPro but you do you.
That said, I’d go for 1, considering you already have that 6th gen on hand in case you need a spare.
Otherwise 3 or 4 (whichever is available on secondary markets for a decent price) and hang on to that Pentium in case need arises. Doesn’t sound like the extra power draw of an i7 is worth it for this build.


Another thing they may have in mind is ATX PSUs. The pinouts on those for the same physical plug vary not only by maker and model but sometimes even by year. So if you get an aftermarket ATX-to-SATA cable that fits just fine in the SATA plug on your ATX PSU, it may put 12v on the 5v and fry your drives or mobo when you plug it in even if it’s from the same brand.
Don’t ask me why there is a voltmeter on my desk.