Am I ready for 300km? I've heard that once you've ridden 200km, the bigger distances are just about proper nutrition and hydration. Is this true? I want to know if I'm ready for a bit hilly 300km, if my current max is 200km.

I did 200km yesterday and was feeling ok afterwards - just a bit tired. After my 200km a week ago I was much more tired and in pain, so there's a progress. 

Is there a big leap between 200km and 300km rides?
If you've managed 200 km without problems you can also ride 300 km. As usual don't ride too fast and keep yourself well fed.
1st big leap is usually between 400 km and 600 km, because after 400 km you will (typically) need to sleep.