I just bought King of Tokyo with the Power Up expansion and we like it a lot. It's a bit strange though as the evolution card titles do not seem to match the monsters.
Welcome to the BBG forum, this is all anyone has been complaining about for months now.
They had to discontinue Cyber Bunny due to legal reasons.
They optionally discontinued Kraken.
There is nothing preventing you from using Evolution stacks with mismatched names. If you are OK with using the 'wrong' cards then you do not"need" to buy Power Up 2nd edition.