At age 16 you may still not be able to do this line of work LEGALY. At age 16 you are required to take a 30min break ever 4 hours (you must clock out and clock back in, if not the company you work for can be fined heavly for each time you dont), and you are limited to the type of equipment you can use, for your own saftey.
I would not worry about haveing your customers sign a contract just agree on a day and time that you will be there to mow and make sure you show up on time. Alot of folks will resept a 13 year old kid breaking the mold and will reward you for your efforts. If you want to buy a good used commercail grade mower you are looking around $300 (21 inch self pro walk behind), keep a rainy day fund and buy a new one when you can no longer keep up with you customers demands or your old one breaks.
North Pasco Lawn Service