When is upgrading to FCP worth the expense (Program and hardware).

What editing software are you using now? If you have FCP already there is no cost to upgrade, just download the current version from the App Store.

Which one to buy? Which one what? If you're getting a new Mac Mini, be sure to have a minimum 32GB RAM and 1TB system drive, you'll be ready to rock and roll. FCP will work just fine on that system.

Can you explain exactly what you have and intend to get etc.