What kind of computer and RAM should I get for 2D digital illustration?

Check the model and age of your Wacom tablet before buying. Very old tablets may not have current drivers for newer versions of macOS, so confirm that your specific model is supported. For the computer itself, 32 GB of RAM is a comfortable choice if you can afford it, especially if you work on high-resolution illustrations while keeping reference images, tutorials, and other applications open. Sixteen gigabytes can be enough for lighter projects, but it leaves less headroom.

The operating system generally isn’t the important compatibility issue; the files and fonts are. Editable text can reflow if you and a client don’t have exactly the same fonts installed, so it’s best to use common fonts or convert final text to outlines when appropriate. Mac and Windows are both perfectly capable for illustration, so choose based on your preferred workflow and budget rather than outdated ideas about one being necessary for creative work.

A Mac mini should work fine for this. The main Adobe, Krita, and common image-file formats work across macOS and Windows, so exchanging projects usually isn’t a problem. I’d consider 16 GB of RAM the minimum, though 32 GB would give you more room for large files and multitasking.