1. Depends on how many DLC package you want to download. Getting everything cost a ridiculous amount of money, several times the cost of the main game, though the exact amount escapes me. The DLC's description and price can be found in the instruction manual, which is located in wiishop channel.
2. No, but the DLC stages are harder.
4. No. that's what DLC are for, or at least that's what they want you to think.
5. Not right now, and probably never will. --- You're a pretty difficult opponent, water. -Kouta Ochiai, Anedoki (Imangascans TL)