i had a gsxr 600 for a couple years, it was the best thing ever. I had not previously ridden dirt bikes or atv's or anything, i took the rider's safety course and got licensed through that. But, I was 23 when i did that and "fairly" mature vs. being 18 and getting one.
Let's be honest, you know what you want, and chances are you don't have a monster amount of disposable income where you can buy a small bike then get bored and then get what you really wanted the whole time. I'd get what you want, and take it very very easy. Stick to country riding and backroads for a month or two until you're very comfortable, then move up to some slightly busier roads and eventually you'll find yourself on highways and such.
Spend money on good gear, get a good helmet, get riding boots/pants/jacket/gloves. and wear them. It's 90 degrees out? Too bad, get a hot weather jacket and wear it. You can't afford not to!
i found the most helpful thing, was to find friends who i knew weren't idiots or crazy or anything, and ride with them. however, you'll find many more people on sport bikes who are crazy and irresponsible, than those who are calm and cool about it