The EX250 is a great little bike and there are millions of them around, so used parts are easy to come by and cheap. Keep it in the upper rpm range and they can be quite fun little bikes. I think you'll get tired of the power in short order if you become a real rider and not a one and done, like many people are. If you take care of it, you should be able to get 100% of your money back selling it to another first time rider when you're ready to go bigger and faster. Keep track of your tire pressures and oil level and I always recommend using non-ethanol fuel whenever possible with carburated bikes. The alcohol in fuel these days eat up the rubber parts over time and if you live in a humid area like FL, when the alcohol mixed in the fuel dries up, it attracts water out of the air and seperates to the bottom of the tank which is what goes into the carbs. Carb bikes don't like running on water., well nether do any other bikes, but with fuel injection it will still run if it is just a small amount.
BTW, you dodged a bullet with the 450 Rebel.