Hardware advice for 2021

Hmmm, what budget ?
I can post the spec of what i am using right now, but it might be bad for public health seeing as graphic cards are harder to find than spare kidneys !
HP 8300 SFF
Dell Optiplex form 760 and up
Fujitsu "corporate" edition are also very good
Well, those are all corporate or enterprise edition PC's, so use much better electronics inside than the "office" or "home" edition ones. Priced from 80 to 150 Euros here.

Optiplex 760 has a parallel port, so if you plan on starting with it, get that. The others will do just fine for ethernet mesa boards, some do have PCI or PCI-E slots.
HP8300 is for ethernet only.
Some Fujitsu also come with a motherboard header for parallel port, so a 5$ cable/bracket is all that is required.
Memory, for the official release of LinuxCNC that is based on Debian 10, 4GB of RAM are enough, Mint with Mate desktop will work fine with 4GB, but 8 would be better. 16GB are overkill for everything regarding machine control, but if you also plan to do CAD/CAM on it, it comes handy.
I never even tested any i3, i avoid them as the plague, but i5 and i7 worked just fine.
Same for the integrated graphics, Intel works in most cases, but i am aware that some chipsets have issues with a Linux driver, even those can be used by blacklisting the offending driver or using "nomodeset" at boot time.
After everything above, you are buying a used PC, so be prepared for disappointment as they might come with a variety of issues, from annoying to plain unusable.
There are 4 used PC shops in my town, i bought over 40 of those old PC's from 3 of them and never ever had any issue with them, the 1 remaining shop i got 4 PC's and all of them had issues and were utterly unusable.

Jake wrote:

It would be nice to be under $1000.

OK
Asus ROG STRIX B550-i Gaming ~200
Ryzen 5 5600x ~230 >>I have a Ryzen 7 3700X
2X 16GB HyperX DDR4 3600 dual rank RAM ~240
1TB Kingston NVME SSD ~120 >>I use a 32GB NVME, yes 32GB !
Seasonic 500W SFX Power supply ~170
Cooler Master NR200P case ~110
Cooler Master MasterCool 240 Lite ~90
4X Arctic P12 PST CO fans ~35 for 5, the stock cooler master vibrate a lot
Optional : Saphire "reference" AND Radeon RX 6900XT ~2300 << I did not pay for this, i did for the rest, took over 2 months to gather the money!
Might want to change the processor to Ryzen 5 5600G or ryzen 7 5700G as they have built in graphic cards that put intel to shame, the first is the same price as the above, the second should be under 300 i think.
Sans the 6900XT, the economy would be
2X8GB DDR4 3200 RAM ~60
500GB Kingston NVME SSD ~55
Corsair 500-600W SFX power supply ~120
Arctic cooler, 120mm or 92mm, ~28-35
The good thing about the fans mentioned above, this is in my bedroom mining 24h, and i can sleep!
.. and work on two big monitors while it is mining. Does not like Freecad and susperslicer though, so i have to stop mining when using those two