I am using an 8yr. old computer and it works great for me!
I am able to play many tournaments (12 to 14)at the same time, across a few different sites (ie. Pokerstars,
partypoker, 888Poker, ACR, GGPoker) and while using PT4 and watching a couple Twitch streamers. Doing all of this simultaneously I have zero LAG issues. My CPU runs at a bit high % once I add in multiple streamers and am considering upgrading to an old i7 cpu I have in storage.
To give you an idea of what it takes to do this, here is what is in my computer:
CPU: Intel i5 4670
Ram: 16GB
GPU (video card): MSI Geforce GTX 1060
500 GB SSD < for Operating system &
Apps.
500GB WDBlack HardDrive < for storage, videos, pictures
You can probably get by without using a GPU (video card) as the onboard graphics in newer chips are probably good enough for
online poker. I chose to install one as an upgrade in my computer (an 8yr. old build) as I'm running 3 monitors (2x 32in. monitors + 1x 24in)
IF I was looking for a computer to play online poker with, I'd be purchasing used from somewhere like Facebook marketplace... either parts and put it together myself, or someone else's used rig (there are LOTS of younger folks out there who are regularly upgrading their computer builds, using them for gaming. An older one from someone like this could be ideal for your purposes).