Question

A simple notepad VST or inbuilt section within Studio one that's used for writing useful notes e.g. key of song, etc

Answer 1

SO5 has a notepad already. Just right-click on an empty space in the console and select "Edit Note". Then you will see a window pop-up. It will have different tabs like Info, where you have the song metadata, Song Notes and Track Notes (notes specific for a single track).

You don't need to use notes for the key of the song, though, just use "Key" menu right beside the tempo.