Shopping for the DM's used PC

A couple of years ago I tried to online-DM for the first time, and we had problems. Turned out, some of the trouble was  because I was using an old PC.  Updated PC, trouble went away. Now a pal of mine wants to make the same jump.  I'd like to help him avoid the trouble I had, but money is an issue.  A brand new PC is out of the question, used is better, but some pieces matter more than others.  There are lots of used ones available, but we don't want to choose by mistake.  Some pieces of the hardware matter more than others but that's about as far as my expertise reaches. CPU?  (My guess: not really.)  Specific video aspects to look for?  Minimum amount, or category, of RAM?  Some magical production age?  I'm guessing off the top of my head but you get the perspective.  How ought one gauge the DM's arsenal? Thanks.

Any computer capable of running Windows 10 should be fine. I wouldn't go older than that due to programs aging out. (For example, Chrome no longer supports Windows 8.1 and older.)

I was hoping for some specifics in terms of hardware capability.  I'm a Linux person; my Windows experience drops off around 1996 or so.  My pal does run Windows, but let's presume that the hardware is delivered bare.  How far back will Windows 10 run?  And/or, what does roll20 need to be usable? Imagine we go shopping at a flea market.  What do we need to make roll20 spin? Thanks.

And: I can imagine (wrong?) a back-office machine, running Windows 10-or-such, to do some specific work where a bottom-end video card was good enough.  If we use that, are we due for disappointment?

Any computer built in the last 6 years or so should be fine. A bare bones video card is fine as long as it has hardware acceleration and WebGL support.  Really, Roll20 doesn't need a lot most of the time.  One of my older computers I still run is an i7-4790k (4th gen Intel) with 16 GB RAM and a Geforce GTX 970 video card. I built it around 8 years ago.  It runs Roll20 just fine.  There are too many hardware configurations to really give you a hard answer. However Roll20 does have this article on it. If you find a machine and you'd like to know if it'll run Roll20 post the specs, we could give you an educated answer. 

I am running on a 10 year old pc with Windows 10, i5-2310 processor, 8 GB of memory  and 2Gb GT 1030 graphics card with zero problems.

I can run Roll20 on a cheap Chromebook. It's a bit slow, but it doesn't fail to perform. Any reasonable PC will do. Some people who have reported lag on Roll20 have very high end gaming rigs. Hardware is a low consideration. More important is internet connection, browser settings, VPN usage, webcam drivers being up to date, OS security getting in the way and so forth.

Hello, Michael J.! There's great advice here in this thread, and we do have System Recommendations in our Help Center that should help.

Michael J. said: And: I can imagine (wrong?) a back-office machine, running Windows 10-or-such, to do some specific work where a bottom-end video card was good enough.  If we use that, are we due for disappointment? I run linux on my home machine as well.  My home desktop is way more than required to run Roll20 in a browser.  The machine I use at work (windows 10) just has a 6 core/6 thread processor with integrated graphics and only 8gb ram.  I log in on it to test things sometimes, and I have no problems running Roll20 on it.  I used to have a cheap walmart sourced hp laptop with similar specs (6 core + integrated graphics), though I had upgraded it to 16gb ram.  That machine would run Roll20 (GM role) + Discord just fine.  I ended up sending it to a GM friend of mine a couple years ago to use until he got a new desktop when his old one let out the magic smoke.  His wife is still using that cheap hp laptop I sent to play. It really doesn't take much to run Roll20 in general.

Much thanks, all.  Now, I shop; later, let us hope, I mock and mock and mock.