If you are keeping your other 40hr+ job then I would say 40 accounts is biting off more than your can chew.
However, if you are quiting your other job and plan on doing this full time then 40 accounts is not enough.
To further help you we need to know:
How old is the equipment? How many hours are on the mowers? And, most important, how much is he asking for the business?
Put the Blades on the Grass and SPIN 'EM brother, SPIN 'EM.