The good thing is you have a customer base to start the process. Problem is you have no free time to Mow. Your going to have to find a employee then to either handle your apps so you can work on building the mowing. Chances are if you stick a employee in your mowing side, its going to be a bad turn out.
Like said Mowing is easy, thats why alot of people get into it. Though there isn't a grey area if your professional or a hack and even that is in the eyes of the customer not in the eyes of your colleuges. When it comes down to recession time, if again not hopeing it will but it will, up by me Fert service would be cut or cut down by residental and commerical vs Mowing would stick around.
You won't see a ROI as high as you would your App side, I found you have to be in mowing for the long haul to get a decent ROI.
I'd think your entry cost should be lower, since you own a truck already and probably own a trailer already. Just have to pick a mower, trimmer, and a blower (again you might have a second already).
Honestly, your from Ohio, I'd hang a plow off the truck and get into snow removal before getting into mowing. It'll get you more year round work.