PC

adleyamanda​  I can find you a monitor for well under $100, and you can get a basic keyboard and mouse for very cheap, especially if they're used.  For the monitors, here are a few examples:

https://www.bestbuy.com/product/acer-ek220q-e3bi-21-5-ips-amd-freesync-monitor-100hz-refresh-rate-hdmi-port-1-4-vga-port-black/JJ8V8Q45XV
https://www.bestbuy.com/product/hp-21-5-ips-lcd-fhd-75hz-monitor-vga-hdmi-black/JJG3J63PTX
https://www.bestbuy.com/product/msi-pro-mp243xw-24-ips-lcd-fhd-100hz-1ms-free-sync-monitor-built-in-speakers-displayport-hdmi--matte-white/J3P7TXL69Z

If you really don't want a standalone computer, that's fine, but you're not going to get quite the same performance unless you go right up to the limit of your budget.  This is the best one I'm seeing for below $600, and it should be able to handle all current Sims 4 packs on high settings, but maybe only barely (depending on how you play):

https://www.bestbuy.com/product/acer-aspire-c24-2g-ur15-23-8-all-in-one-desktop-amd-ryzen-5-7430u-16gb-ram-512gb-ssd-windows-11-wi-fi-6-bluetooth-black/JJ8V8HGLV9

This one is $600 (before taxes, of course) and is faster than the ones I linked earlier:

https://www.hp.com/us-en/shop/pdp/hp-all-in-one-desktop-next-gen-ai-24-ct2055m-pc

I checked a number of different sites and didn't find anything better, unless you count a few options from questionable third-party sellers on Amazon.  And that's not surprising given the usual price for this tier of hardware in an all-in-one desktop.

If you're willing to buy a laptop, you could currently get a proper gaming machine for $550 or less:

https://www.bestbuy.com/product/lenovo-loq-15-6-full-hd-gaming-laptop-amd-ryzen-5-7235hs-16gb-memory-nvidia-geforce-rtx-4050-512gb-ssd-luna-grey/JJGH3SKKHG
https://www.bestbuy.com/product/hp-victus-15-6-144hz-full-hd-gaming-laptop-amd-ryzen-7-7445hs-16gb-memory-nvidia-geforce-rtx-4050-512gb-ssd-mica-silver/JJGH2L8JVV/sku/6623881

These are usually in the $700 range and are decent-quality laptops, no frills but not low-end either.