Make sure you use a quality grass seed mix. Up here we call it "grade one"
Grass seed usually has at least three different families of grass in the mix, here.
That way if one family gets a disease and dies out, the others will take its place.
I believe they do the same with wheat farming and potatoes
It sounds like you have researched your project.
Some-one mentioned what if another company comes in after you and sprays....
You better find out from the client and/or put a disclaimer into your contract.