The best hardware choice for running macOS/ OS X 10.9 is the Intel Haswell based systems. There are many guides on this site for that era of hardware. Intel 8th and 9th gen chipset hardware. Here's the older BG.
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We've found that the Dell Optiplex pre-builts work great with 10.9 through to the latest Sonoma. Gives you a lot of options for the OS choice. You can also run Windows 7, 8.1 or 10 on these with full driver support.
The Dell Optiplex 9020 and LG 4K IPS Monitor ____ Components Dell Optiplex 7020/9020 Mini-Tower (MT) Amazon - Newegg Dell Optiplex 7020/9020 Small Form Factor (SFF) Amazon - Newegg Dell Optiplex 9020 Ultra Small Form Factor (USFF) Amazon - Newegg Dell Optiplex 9020 Motherboard...
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They won't be compatible with the older 10.7/10.8 OS.