dealers do most of there business in the summer so you have some more wiggly room in the off season with price. Supple and demand. Some dealers get back repo or even sell floor models that are less then a year old. if you stop by a dealership and talk to them, let them know what you are doing let them know your price,and ask for advise.
at 14yr old i would not buy anything new
As for brand name everyone here will tell you something different, it boils down to your prefernce. Different machines have different features. Some machines are easier to use then others. You can ask your dad to try out the machines at the dealer to see what is easist to use. Do some reseach on your own. Most guys here list what type of gear they use. Make a list and narrow you list down based on what you can afford and what is avaible in your area. I would not buy anything off of craiglist or ebay unless you know a mechanic. If you are just doing 5 lawns a week your toro will last you a while. I have had my 21inchToro for over 5 years. Good luck and dont be affaird to do your own research.