Hi Oliver,
I've found a good practice is making an USB bootable installer for every OS my Macs go on.
https://support.apple.com/en-us/HT201372
Just buy a load of cheap USB sticks and reserve them for the OS Installers.
1. It's a lot faster than downloading from the internet
2. You can reinstall the OS pretty easily, especially if you've got a good Time Machine backup.
Andrew