Welcome. One of the things that often surprises new riders is the issue of insurance. Before you dish out for a new (or new to you) bike, make sure you've lined up insurance you can afford. 300cc bikes are typically cheap but for, say, a 17 year old male, new rider living in Toronto it could still be prohibitive. You don't mention your age or location so I'm just throwing this out as an example. If you're 50, have 30 years of clean driving on a G (well, GM2 now) and have a car or two and a house with insurance company 'X', you might get bike insurance fairly cheap.
Sport bikes -- even 300cc ones -- are sometimes frowned upon by insurance companies. Some consider anything with "Ninja" in the name to be "blacklisted"; they think if it's a "Ninja" it's just gotta be like a ZX-10RR. If you get grief like that from your insurer you might consider a different kind of bike: Instead of a CBR500R, think CB500F. Instead of Ninja 300, think Versys 300. Or maybe a cruiser; you can generally get quite a few more cc's for your insurance dollar in the cruiser/touring category than in the "sport" category.
Check with your insurer. If you find they're not willing to help you out, look a the insurance forum here for brokers and agents who might be able to help you more.