Here's the best low cost choice to combine with an i3-8100.
https://www.amazon.com/dp/B07BQ9XQPP/?tag=tonymacx86com-20
Has onboard DP output if you ever want to upgrade to a 4K monitor in the future. I also like that you get an Intel GbE LAN and dual M.2 for an NVME boot drive. All for $84.99 ? That's an incredible deal. If you use Mac Mini 8,1 SysDef it should be fairly easy to get everything working quite well.
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Compares quite favorably to an 8,1 2018 Mac Mini base model. About 1/2 the price if you already have a case and PSU. $377 for a new highly compatible with macOS, 8th Gen system, what's not to like about that ? The only two things you won't get is 4 TH3 ports and the ultra small Aluminum Mac Mini case in Space Grey.
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