I’m with spaz in that if you know you are going to speed and get yourself into trouble, owning a bike that is a) less capable of those speeds, and b) feels “faster” at low speeds (like the 250R) is a good idea.
Beyond that, I think the GS500 is probably the best bet. It is slightly more restrained/civilized than the Ninja 500 according to the reviews. It looks sexy, and you seem to be able to get a good deal on a new (or almost new) one. Though in all honesty, I don’t have any idea what a good price for one in your area (or even my area!) is.
The 500 is certainly a bike that you won’t out-grow in the short term.
As for track days, consider saving up and attending a track day school. Around here these cost $500 and include all gear plus bike for the day. They’ll teach you proper high speed cornering and general track riding rules and skills. Certainly not the cheapest one day holiday, but training is the cheapest performance mod you’ll ever get
Of course, once you get used to getting your speed fix on the track … well, it can be an expensive habit from what I’ve heard.