For external data, should I create custom table or new entity type?
It's really up to HOW you are going to use it. Just listing it? Will you extend it with new fields? How likely will be problem be more complex in the future?

Custom entity is a definitely good thing: it's not just about views, but lot more (entity hooks/events for example). Don't worry about performance yet, since you didn't mention anything that could be dangerous. 
Still regarding simplicity the table solution would be still a winner.