It sounds like a straight-up cash purchase of a "turn-key drone" is your ticket. Otherwise, I'd recommend some kind of "DIY framework" - a drone platform that's taken care of the aerodynamics, controls and fuel tank and radio controls for you. Then you just tweak it to match your exact need.
My advice: whether, you DIY it or buy it outright.... don't skimp. Walk into this knowing you're probably going to spend twice as much as your initial estimate, if you can budget it. A semi-autonomous LONG RANGE drone is NOT cheap. A 20km bare minimum range puts this project into a semi-professional to professional level. Most "hobbiest" drone projects or commercial products couldn't even spit at the kind of quality and scale needed to perform such a task.
If you decide to buy something... look at commercial surveying drones. They have the range, the quality and the sophisticated integration already taken care of for you.
Do your homework upfront, buy it right the first time, take care of it and maintain it properly and it will give you YEARS of little or no issue service.