How much are you paying for it and at what rate? I got my '16 300 brand new right after they came out last year for $4500ish OTD. Qualified for 5% $0 down. ZERO miles on the odometer; I HIGHLY recommend you do the break in yourself, do you really trust other strangers to do it (it sounds like a demo bike)? They also did not require full coverage. Today they are still worth the same, lol, my local dealership does well with sales. For reference, sale price of the bike was around $3600, then you had the new vehicle charges such as delivery and setup (which usually add up to $500-1000 by the way). I think the bike was around $4,050ish (after these fees) + TTL. If you buy a used '13-15, you might be looking at $2500-$3000 + TTL (which is a hell of a lot cheaper than what most dealerships want for a brand new bike).
That's another thing, my personal advice from my lovely experiences, if I could do it over again, I wouldn't buy new. Take the extra $1k-1.5k and buy GOOD gear, that saved my life once before (doctor said without my jacket I'd have a punctured lung and wouldn't have made it to the ER), and I'm sure it'll save it again some time. There are so many beginner bikes to be found for under $2500; sure it's old, but it's something you can sell when you're done with it for what you paid, and it's something that, if you drop it, you don't get P/O'd. The 300 is a great bike, and I'm sure you could find some decent ones in the $2000 range. I know it isn't what you want to hear, but it is my biggest regret with my bike. I didn't even have the chance to take off the temp tags before someone totalled it; here we are a year later, and my attorneys are still struggling with the case (I am not at fault, the other driver was, complex situation, bad insurance company, etc). My brother was also in a similar accident just recently, and despite being NAF, it appears that it is going to be a similar struggle for him. Oh, while we're on the topic, buy a helmet camera!!
Here's a listing of used 300s and the price to expect from them:
https://www.cycletrader.com/Kawasak...8344&model=ninja 300|764858052&trim=300|2851&
If the link didn't work, I just filtered Ninja 300s for 2014-2016 then sorted price low to high; the cheapest is $2k. I might recommend the ABS, but that is up to you. Most of these will negotiate.
PS, not too sure what exactly I am allowed to say here, but to be honest, if you've already practiced with a 250, I'm not sure that I'd recommend such a small bike to start on. Especially, if anything, a brand new one! Everyone I know who started on a 300 sold it within 3-6 months of ownership, even people who swore to keep it for at least a year. People who have ridden once or twice in their life on a buddy's dirt bike, and then at the MSF course. Since mine is totaled, I'm trying to rebuild as a track bike. It's a great bike, it just depends on what you want to use it for; if you want to practice track skills on it, then go for it, but for learning to ride on the streets, if you already have experience, then why not a 650? *MAYBE* a 636 depending on how much experience you have with your 250.