Here's my experience
I bought my R33 GTST within two months of my 18th b`day... and boy did i thrash that car, almost died in it. I sold the car 11months later b'coz i knew if had kept it any longer it would be my end. It truly is a miracle that i never got caught speeding but that doesn't mean it wont happen to you either. I was speeding pretty much every time i got in the car, doing wild insane stunts, switching lanes on the freeway etc.. it was crazy Until i once had a near death experience, could of died but i guess it just wasn't my time yet and that woke me up so i decided to get rid of the car and it was a the best decision i made. So i went out bought something reliable and near new Pulsar and had been driving that for almost 3 years till recently sold it.
So what I'm saying is, no matter how much you say " nahhh i wont be like that bloke, i'm going to behave " we both know you are only kidding your self. When your young, you do all these crazy shit you find amusing and mostly come from peer pressure. It's your money buy what ever you want but these are just an advice from those who once have been 17-18 yr olds and has been threw it, experienced it all. If you know for a fact that you will behave when driving then go for it man, but from what I have gathered from your posting tone I highly doubt it.
Once you mature up to 20-22yrs old, you don't even speed anymore, you just cant be bothered to speed.. Like my self, i know when to speed and when not to. During week days i stick to the speed limit and drive calmly but weekends, on open roads, no traffic.. its a different story!