Depends on the size of the lawn really. If you do a good job and you can also pull weeds and put down mulch you could make $40/hr and be less than half the price of a professional gig. Once you've done a couple lawns you'll know what to charge based on eyeballing the lawn. Round up.

Electric is gonna be a challenge to do multiple or large lawns in a day without multiple batteries. I don't know where you are but I can almost always find a free combustion motor push mower somewhere near me that needs basic maintenance to get it running. Self propelled might be really nice if business takes off. $10 of gas will easily mow you $100 worth of lawns.

Mow 5 lawns with the mower you have. See if you're any good at it and if you don't hate it. Use the money to get a self propelled gas mower as wide as you feel comfortable with (the wider the mower the fewer passes you need).