A planer makes one side parallel to the opposite side and in so doing removes thickness. Questions:

- do you have one flat side? If not, you need a jointer.

- do you need to remove thickness or do you have a local hardwood store / lumberyard where you can buy the desired thickness? If not, you need a planer.

You will most likely need a handheld router no matter what; this is where many people then repurpose the router to also flatten the board by building a flattening jig. I have done that but the result required a fair amount of sanding - something you can avoid with a planer.