Are you taking MSF with NCMT in San Lorenzo? If so, their fleet of eliminators are all
125 cc. So if you felt good on those you can still double your displacement and upgrade to a 250 when you get your own bike.
This is truth. I took the same NCMT course on the same bike two years ago, bought a Ninja 250 shortly thereafter, bought a
second 250 five months later, and am still learning how to improve my technique on them. But it depends on what your personality and situation is - not that my advice will change, but if you're the type that's got deep pockets and is trying to compensate for something I don't know if it'll sink in.
Still, taking MSF and having the humility to post about it on BARF (with key phrases like, 'doing my best to be smart and responsible') sounds like you're off to a great start. So since the next step is getting a bike to practice on, my first piece of advice is that whatever you get, get it
used. And while bikes are generally more pricey in the warmer months, I've seen some especially
good deals on 250s lately (and no, unless you're over 300 lbs., you do not 'need' the EX500 instead). They're really cheap, they're a whole mess of fun, and they're a lot more graceful in letting you make most of those beginners mistakes. Then, if you find out you're really an enthusiast for a bigger, shinier, prettier bike... you can sell it for about as much as you bought it, and you'll at least be better prepared. But if you find out that your enthusiasm is more for the genuine skill, I think you'll be holding onto one for a while. Good luck.