(re)making an airport.. Where to start?

[Softpxl 4]

Okay so I have been using ADE for quite some time now to correct some of the runways that don't have the right runway numbers. but recently I have been thinking of making my own airport, or rather kind of remaking/updating one.
But before I mess up the whole project, I was wondering what kind of the work stucture is when making an airport from scratch?
Where do you usually start with?

Are there any other programs I can use besides ADE?

Is there a way to add runway slope to the airport?

how do I make sure the airport and the surroundings are the correct heights?

How do I get the right ground textures for the ground, taxiways, runways etc?

 

The airport I wanna create is Keflavik International(KEF/BIKF).
Currently there are only 2 addons of Keflavik that are kinda okay, sadly the AS version is not fully supported for P3Dv4 and the Orbx version is not very detailed. But both are still out of date anyway.
Since the airport has got some extra parking spaces and some changes to it's layout, I wanted to make a up-to-date version of it. 

Any information can help so please feel free to share your experience and tips.