I would like to buy a Mac mini and customise it.

though if you customise it youself then you will void the warranty

The only customization you will be able to do, once you selected with Apple what upgrades you need (and ok to pay for) will be what you externally connect to it: drives, monitors.

Plan carefully, depending on your needs and habits, for what will be ideal “internally”, then plan what can be easily used “out of the box”.

See Buy Mac - Apple (AE) and “Ask the Mac Specislist”