If you're planning to buy the latest new hardware, you're probably already close to too late for SpinRite 6.1, which will not support UEFI Class 3 motherboards. I donno how you can know which motherboards support a CSM and Legacy Boot, and which do not. Intel has indicated that UEFI is the future, starting in 2020, and Legacy Boot is no longer to be supported in 2021. (See
https://forums.grc.com/threads/the-end-of-legacy-booting.165/ )
SpinRite 7 is planned to support UEFI booting, but it is probably still a couple of years away.
Thinking of PC's I have to hand, that have cost effective hardware, I have a Ryzen 7 2700X on this motherboard
https://www.asus.com/ca-en/Motherboards/PRIME-B350-PLUS/ and it has a CSM and Legacy Boot and works well with the current AHCI test program. (Even allowing the M.2 to show up in the BIOS.) It was a value board, and it's older now, so it might be hard to find, donno about that.
Since the MOBO provides video outputs, you could use an APU so you wouldn't need to invest in a standalone GPU. I think this machine would be quite happy to run a Ryzen 5 2400G or even a 3400G. That wouldn't have it being a killer gaming machine or anything

but it should run SpinRite just fine.