Welcome!
It's funny... after about 2 months of riding the ninja 250 you start to get bored of it... However, if you begin to unlock the potential of the 250 it becomes revitalized. First off, riding the bike and keeping it close to it's max power around that 11k rpm range and you'll have a blast! Also take it down some twisties! It's very fun! On the twisties you cannot go very fast or you'll die lol so the need of a bigger bike goes away.
If you only commute on long highway and straights, etc. Then yes... you will get bored... I will admit that to me personally going from ninja 250's for 5 months to a ninja 300 the extra power will be noticeable and the ninja 300 actually pulls you a little bit which is awesome....
Regardless, I think you are in the right mind set, get the cheap bike you can AFFORD. Once you learn to ride and have fun, then sell it for what you paid for it and get the bike you want.
Good luck
