Is it necessary to have seen Part 1 in order to understand Part 2?
No. My wife and I saw it yesterday, and she had neither read the book nor seen part 1, and she understood what was going on. You will understand more of the understories and the characters having prior knowledge, but the movie was done to be able to stand on its own.
I’d suggest you read the book. Watching the movie afterward is a nice treat, but buy the book.
You don’t need to see Pt.I to have Pt. II make sense I think. Pt. I is on Netflix and Netflix usually has a free trial period available, so you coudld watch it that way.