I would say a Ninja 300 is a terrible choice for a first bike for the same reason a CBR500 would be: way too expensive, no used examples, and you will lose thousands if it turns out to not be the bike that works for you. Being brand new and in demand, dealers will be less willing to waive fees, you'll probably be paying close to full MSRP, and will suffer the full force of depreciation that both things entail. Won't be as much as a CBR500, but it will be damn close.
If you absolutely must get a brand spankin' new bike with 0 miles on the ODO, go for a left over Ninja 250 or CBR250R. Dealers are more willing to waive the standard fees for left overs, and they can go for thousands under MSRP. If you stay under 4 grand out the door including tax, you can limit your loses to probably about 1000$ or so dollars if/when you decide to change bikes (as opposed to 0$ with just buying a cheap used 250 in the first place).