for that price, a slightly oldschool second hand bike would give you a better bike than a "300 new bike. Unless the bike has been totally ragged, you should spend less time fixing things than you would with such a cheap new bike.
something like this
Sure it's not a modern bike with fancy new futuristc geometry or frame design, but those frames do ride well, especially coming from a mtb background. I am also a firm believer on riding a short bike if you are new to the sport, it will actually teach you to "ride" rather than hop about all over the place which most modern bikes are best at.
Some people with say get a 20", but I would only agree if you were from a bmx background or really small/young. Otherwise stick with big wheels and get learing to ride over stuff and have fun!