Generally speaking, the starting point for designing an airplane is deciding its purpose . Do you want it to go fast ? Go slow ? Glide ? Carry cargo ? STOL ? Be maneuverable ? Be stable ? Carve ? Skid ?

Once you know what you want your airplane to do, then you can sellect an airfoil. And once you know the function of the airplane and the size and weight of the airplane, then you can sellect your power plant.

So your first question is: what do you want the airplane to do ?