Editing spells, skills, units, etc.

Personally, I wouldn't recommend changing anything. Everything is perfectly balanced.
If you need to change something, you can use the map editor or cheats; ultimately, you can play on a different difficulty level. There are plenty of options. There's no need to change the game itself.

I have a sad experience with editing the game "Stars!". It also has an editor; although its capabilities are partially limited, it still allows for a lot. However, playing becomes worse. Especially considering the huge amount of time spent on the game.
I wouldn't recommend it.