I think it's a great idea! I use Under Armour, which would cost you about $75 shipped for the top and leggings. You could get the off brands, but I hear they don't hold up so well. Either way, if you go the UA route, you'll have to replace it after a while (snags, armor bites, etc). If you're on a budget, you could get a Hasbro E-11 and modify it. It's cost effective and I've seen some done that look incredible. You can usually find someone who knows someone who can get you the parts.
As for being under-aged, you can still have fun with it, even if it's not at official 501st events (conventions, costume parties, etc).
It's a hobby, like any other. It's best to take your time and enjoy what you're doing. Good luck and we're here to offer tips! Well, at least the other guys are, I'm still new to this stuff myself.