What is a recommended input mic for Mac mini?

Are you planning on recording music, or are you just using it for video calls? For videoconferencing, any cheap dollar store headset will do. If you have Apple AirPods, those will work too.

For an entry-level musician, you can try getting a dynamic mic such as the Shure SM58 or the Audix f50 (I use the latter), which has an XLR jack which plugs into an audio interface like the Steinberg UR12 (which I use).

I use a Logitech C270 webcam for £20.

It is only 720p but the video quality is better than many 1080p models. Much much much better than my 2017 iMac.

The mic in it is also perfect for anything other than out and out music recording.

There are literally dozens. Which will suit you best will depend on what you wish to record and of course your budget. If you can post details about your intended use and needs, someone can probably offer opinions.

Regards.