I would say talk to all the neighbors you can.
We were going to focus on the ones that have overgrown yards. I figured he could make more for his time mowing yards.
This is a fascinating topic. We have seen on here how when some lawn care business owners focus on only overgrown lawns, they find the lawns are overgrown for a reason.
A lot of times that reason is the home owner does not care or is broke and either way, those are bad customers.
So I would say don't limit yourself to one specific group like that to market too. You may think at first the group you have chosen is a slam dunk only to find yourself later sitting around waiting for the phone to ring after spending a lot of time marketing to one group.
Keep us posted on how it all goes for you.