This is also from a novice speaker builder, so verify everything at least twice.
MDF/Supawood is best according to many speaker builders. It has no cavities, has good damping properties and is uniform throughout, so you won't get one light plank and one heavy plank. You can always do veneers etc on the mdf.
Driver size isn't as important as the properties of the driver, these suit a recommended enclosure size and type. I would select a ballpark size in litres and narrow down driver selection that way, and then budget and then recommendations from the other diy people and internet. This is just what I would do, not necessarily the right way round.
I think it is best to start with the bass driver, and then select the tweeter from there.
Oh, even though I didn't follow my own advice on this one, well sort of, most other builders will suggest a proven recipe for your speaker, ie drivers, enclosure shape and size and a crossover diagram.
Like a recipe on Zaph's website:
http://www.zaphaudio.com/
