I would recommend the one you love.

Go to a few bike shops. Sit on a bunch of stuff. Listen to advice. Most bike shops also service so anything you buy, they're likely to get money out of. They may have bikes, too. Either way, fall in love with something, be it a style, a brand, a color, whatever, and find that.

Know that you either need to learn basic maintenance or you need to pay someone. Motorcycles require more fiddling than cars by an order of magnitude. If you enjoy it, this is great. If you don't, it's a drag. Fortunately they're hella easier to work on.

There's this trope amongs the Internet motorcycle community that OH GOD DON'T RIDE ANYTHING OVER A 250 and it's purest bullshit. You'd rather be on something that will get out of its own way because motorcycles don't brake as well as cars and they don't turn as well as cars so the only thing you can consistently do to avoid a problem is accelerate. Combine that with the fact that the secret to surviving a turn you underestimated is throttle, not brake, and "underpowered" isn't where you want to be. That said, you probably don't need a Hayabusa.

If anything, I'd choose something that you can afford to lose, and I'd choose something that won't be worth nothing a year from now when it's time to move on. Most communities have a rotating pool of Ninja 250s that go from one learner to another. Those are fine. Pretty much anything Japanese will be ridden sparsely, dropped a half-dozen times and likely kicking along dandy.

European bikes retain value much better than Japanese bikes.

You will likely get a better deal at the end of the summer than at the beginning.

Anything to add, goobster?