Pretty much anything from a 3-6th/7th gen i5 machine should be very accessible right now, and companies are always tossing Dell towers that are boring but reliable machines. I've had good luck with Win10 on a 2nd gen as well, but you probably want to get as modern as possible.
I usually try and get a couple when I do a project like that, RAM and accessories all go into one machine whereas the second is spare parts. You'll want to pick up a hardware COM port board, and it should be good. Avoid the shit you find on Amazon and eBay that are "refurbished," those are essentially checked to see if they turn on and that's it - they're generally in bad condition because they just shoot them with shit black paint.
You could probably get away with an i3 but those are almost as bad as celery processors.
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