One you start you can't sop! LoL

Gmax can look very confusing at 1st ??? but you will find in no time you can produce great objects. Work through the tutorials with Gmax 1st. They are not FS related objects but will give you most of the knowledge you need.
Simply, you create the 'grey' object in Gmax. You give the object the real feel using Textures which most people generate from photos now. These are applied to the grey box. You then export you object as an mdl object from gmax.
There are many ways to place your object but I find Arno's object placer very easy to use. You simple slew to were your building needs to fo go in FS9, the obplacer reads were you are and you simply place the object.
I use AFCAD to do the runways and taxiways etc and other programmes to place trees, exclude autogen items etc., but one step at a time.
There is also scenegenx and runway12 but I have never used them.
Have Fun, Brittair