New Mac Mini

Duet Display is fairly nice... but it's not intended as a primary display. The performance was never really good enough for my needs.

If your goal is to use the Mac mini primarily for the occasional desktop app, you'd likely be better off just setting up remote desktop and using a client, like Screens, on the iPad.

Not out of the box, you need to set it up with an actual display and purchase some 3rd party software

How to use an iPad or iPad Pro as a monitor for your Mac