I think its a brilliant idea as long as you can financially support it all the way through. I wouldnt recommend driving it until your 21 due to insurance prices amongst other reasons such as experiance etc.
Don't let anyone put you off, but make sure you take into account everything that has been said on here as it is so so true
Like you i was only young (15/16) when i wanted a supra... i finally managed to buy mine when i was 20 and insured it from my 21st birthday
A lot of my friends drive starlet turbos and they are great fun to drive, awsome handling and fairly cheap to run and insure (compared to a supe anyway!) so i would say your best bet is to go for somthing like that!
Oh and don't underestimate the power of a starlet thats been played with either.... my good friend has one running at 342bhp at the flywheel.... leaves me for dust once he gets his grip
good luck...keep us updated dude