The plugins and hdris should be easy to find.
For the textures, don't worry that much. It's a common habit to hoard everything that "might be useful in the future".
I stopped(well, heavily decreased) my habit of hoarding useful stuff and now only get/create it when I need it.
I'd say: concentrate on things that allow you to create your basic needs first and work yourself up from there.
And don't forget that, the most powerful tool you have is C4D. For example, you can easily create trees and other plants using the hair system alone, dynamic to the wind and other forces.(just search on youtube and you'll find lots of tutorials)
Also: With great power comes great responsibility!
heads up!