[sokoloff]

I don't know if there's a site focused on that (would seem like a lot of work for little payoff), but I can vouch that most office mid-sized towers will take a graphics card and most will have excellent cooling. For the Dell 9020, you need a power supply adapter ($14 on Amazon, $4 on Aliexpress; the motherboard takes a custom connector) and a replacement ATX power supply if you want to use a high power GPU. (For other motherboards that have a standard 20+4 ATX connector, you still might need a replacement PSU, but won't need the additional adapter cable.)

I've found Craigslist and FB Marketplace to be a better source, since shipping is a killer (but I'm in easy driving distance of Boston; if you're far away from a major office city, that might not work as well).

The i7-4790 Dell 9020 can't do TPM 2.0, so a few games will be locked out by anti-cheat mechanisms, but you can drop an 6650 XT or RX480/580 or cheap nVidia card into one and play the Minecraft/Fortnite/Rocket League and other light/medium games just fine, especially at only 1080p.

I probably wouldn't bother with a half-height R5. The Intel HD 630 iGPU will outperform an R5 and the Intel 4600 iGPU is 2/3 the performance of an R5 430. if you're out shopping for something new (to you) to buy, I think you can buy a computer with a better iGPU rather than buy a lesser CPU and add an R5. (If you can swing it, I'd probably target at least 8th gen CPUs at this point. Or target 4th gen if significantly cheaper in your area and plan to upgrade 3-4 years from now.)