I find a used Dell 780 makes a really fine CNC machine for MACH3!
It has both USB and Parallel ports, is very fast, usually comes with Windows 7 installed, has a really small footprint, is widely available on eBay for under $100.
Have 4 of them running all kinds of stuff, a great little box, very reliable and works well.
You can use any newer PC... 32 or 64 bit.....
But you will need a motion controller like Ethernet SmoothStepper....
I am using Win 10 Pro, x64 running Mach3 in Compatibility mode( Win Xp)
A good refurbished Dell with Windows 7 32 bit makes the mills run smooth as silk. All you need base line is a paralell port. If the computer doesn't have one, they are about $10 almost anywhere. We used to always suggest XP... and set up a lot of computers for our customers to go with machines. When XP vanished, I set one up on Windows 7.... I was in shock how smooth the mills run on Win 7. We have moved several customers over to Win 7 and had "0" complaints about the setup.