There should be red tiles on the floor of the things you should be removing (without having to remember everything that was just said to you) and when you go to build something in it's place it should pop up with an interface that tells you what materials you will need to build a new one. You will also only use the planks for the level you selected when being assigned a contract, for example if you selected the 70+ tier you will only use Mahogany planks and will not need to bring any other type with you on your contract journey.
I'm happy to pop your feedback down and send it over the developers in the game update review meeting tomorrow of course!