The Ninja 300 is plenty of bike. You don't need a 500 (which 500?). If you were well experienced, then I'd recommend a 600 SuperSport but that's not the case.
I have an RR and a 300. I can still have fun on the 300.
Buy a 2013 used with low miles. Should be able to pick one up for $3.7k that hasn't been dropped and is low miles. Brand new will cost you 5800 with tax title and licensing through a dealership.