There is free/open source software that will compute viewsheds for you, and data downloadable from the USGS. In times past I did a lot of this but really you just need to go look with a telescope or a camera on the end of a long pole. But note that the work you do to figure out if you can provide service to a prospective customer can easily exceed the lifetime value of that customer!
The Post Office (in the US anyway) will mail a postcard to every household in a defined area for reasonable cost.
Remember to get good insurance if you construct or climb towers.
Honestly I really question your sanity though. Think of all the people who decide they are going to open a restaurant: we shake our heads at the large subset of them who have absolutely no restaurant experience. We know that in order to make money at a restaurant you at least need to absolutely nail the whole making food that people like to eat and making the place pleasant and inviting and all that (execution). If you don't have that, you're doomed for sure. And then some large proportion of even the experienced restaurant folks still fail -- we look at their efforts and shake our heads "well they should have known this town doesn't have a big enough market for a fine dining place", and so on.
As someone who does know all about how to build and run a wireless ISP, I'm reading your post like how most folks look at a doomed new restaurant opening.
Sorry to be a party pooper but I figure I may save you some $$$. I think at least you need to be doing this in a place where there is _no_ competition besides satellite. If there is even just regular LTE service people will scrape by with that rather than pay you for better service. If there are traditional providers (Coax/HFC, DSL) then forget about it.