If you want to make a career in game development, then decide first what's going to be your forte. The field of game development is split mainly into programming, graphic/art design, music creation, and level design! Though you need knowledge on all these, you need to have your specialty in a single field to get a good job.

Anyway, I don't really recommend spending on mx2004, as it is quite obsolete when considering the future. I would suggest take the free route......
1) GIMP: for your sprites and arts.
2) FlashDevelop: To code and compile your game.
3) FlexSDK: to support your FlashDevelop.
4) Audacity/Wavosaur : for music creation, or you can get thousands of free music on the web, and most of us use those!

If you get the above, you won't have to spend a single yen, and you can learn perfectly. Also you can create the most powerful games possible with flash today, provided, you learn as3!



As for the idea, why don't you pull out "chopper" from one-piece! And there are enough weird characters, giants, e.t.c to make a cool game!