30 kg of gravel should be enough.
As has been mentioned, it's important to boil or bake anything you put into your tank to kill any living organisms on it. This is a lot more important than 'scrubbing', imho. Also, take some vinegar (a very simple acid) and pour it on the rocks. If there's any fizzing, it means the rocks are soluble and can dissolve into your water over time, releasing undesirable minerals. Make sure you rinse the vinegar off well! With wood, as has been mentioned, unless it's submerged when you find it, you need to 'waterlog' it, or soak it until it sinks. Change the water a couple of times a day while you're soaking it, and you should get rid of most of the tannic acid. Tannins aren't necessarily bad in small quantities (sometimes sold as Blackwater Extract for soft water loving fishes), but unless you're experienced with altering water chemistry in this way, it's best to make sure the soaking water is clear before adding the wood to your tank.