I'm going to respectfully disagree with the idea that this will suit your needs just fine. My fear (and I've seen this happen time and again) is that you'll buy that bike (or something equally crappy) and hate it because it's rough and unreliable. Then you'll have this impression in your brain that riding a bike isn't fun and you won't ever ride again. When it actually is great fun, as long as you have half way decent equipment. I really think the extra $200 (or $400 if you go to a bike shop) will be completely worth it.

If you really only have $300 to spend and nothing more, you should get a friend who knows bikes to help you find one on craigslist.