Learning Japanese & Korean

I am highly interested in learning both Japanese and Korean. I’ve always heard that if you learn Japanese first, you will eventually find it easy to learn Korean. So I was just wondering:

1. What are the exact similarities between the two that makes learning Korean easier with a background in Japanese? Is it grammar? Sentence structure? Which of these specifically?
2. What level of Japanese would I have to reach in order for Korean to be ‘easier to learn’? Would learning N5 Level Japanese be enough?

Thanks in advance!