I work in higher-ed IT and love these computer discussions. I support and use Macs (iMacs and Mac Pros) but also run Windows 10 and Deepin Linux as either VMs or natively, depending on whether I am sitting at a Mac or at my Hackintosh.
Random thoughts:
-OP has very decent specs on the "slow" PC. I also vote for the fresh install. No need to spend a penny there. Sounds like Windows cruft.
-Thanks for the heads-up about the PCIe NVMe SSD option. I hadn't given them much thought as I'm not currently building a new machine as the Hack is running fine and I have others. But definitely something to consider.
-While I love Macs, the last one I bought new was in 1997 (PowerMac 7300), and a used G4 tower off ebay a few years later. That's it. I would not spend my own money again on a new Apple device (price/performance, as noted by other earlier). Of course I'm not still using those machines! I've used older iMacs over the years. At home I currently use a maxed-out 2013 or 14 MacBook Air which was given to me by a colleague (it was actually on the way to electronic recycling, something to do with a cat and a large drink...). The screen is all cloudy but I use it with an external monitor and kb, and it works great (and is actually clean inside). I was also given a 2007 Dell thinline desktop (Pentium, 4 gigs RAM) which I also put Deepin Linux on recently as a spare station.
-If you must buy a Mac, get a used/refurbed iMac (2010 or newer) and replace the HDD with SSD and max out the RAM. Avoid the Mini though.
-Building a computer is the best way to go, regardless of which OS you prefer. At this moment I am sitting at my triple-boot 2011 build Hackintosh in a G5 case (i7 2600, 16 gigs RAM, Gigabyte Z68 and a bunch of HDDs and one old SSD) which I bought from a friend cheap and made changes to, running Deepin Linux which I recently installed (please Google it and check it out). The Hackintosh also boots Windows 10 and OSX El Capitan. It's ugly and noisy but it's fast and efficient. It's my "play" machine" when I feel like tinkering. The video card limits the resolution in OSX and Linux but Windows looks normal.