I would seek out about a dozen clients who cared about carbon and hopefully had small lawns, rich aging hippie types. I would charge about twice what normal lawn service costs; the whole group would arrive and cut lawn and do any other work en masse, otherwise I think it would take too long.
The overall goal was to have enough work to have 1 full day's hard work each week, for which each person would earn about $200. I wanted to design the business explicitly to provide a way to pay the cheapest rent in my area (half the cheapest two bedroom apartment) and a little left over for food and etc.
I wish someone would try this. However, the person who actually starts it for the first time will probably find themselves working full time for quite a while, and it won't be a part time backup job for them.
A side note on lawn mowers -- they are almost all not work horses or indistructable, particularly the $50 ones you will get used. They are garbage. Briggs and Stratton has a good reputation, but I am not sure they are currently living up to it. A good rule of thumb is to get the best ratio of engine displacement to cutting swath you can get. If you get the used ones, get three and expect to be constantly repairing them, which will be fun for a while and then get old.