Doing scenery (terrain) isn't that difficult, mainly because there is no right or wrong way to do it so to speak. The only thing that may mess things up is your imagination or lack of it as the case may be. That aside, anything is fair game. Even water for rivers, lakes etc is pretty easy nowadays so long as you can use a paint brush and paint to paint the river/lake bed or to color the river material you chose to use. Envirotex Lite is an excellent medium for water by the way.
As for the structures I think if your just getting into the scenery and model making side of things I'd probably start out with something DPM Kits. Styrene/ABS relatively well made and easy as heck to assemble. If nothing else, the parts will give you the appropriate sizes for walls, doors and windows etc, something to use as a template if you will. I'd then jump to Laser Cut Kits, a bit more challenging but a lot more fun to build.
As said, you can buy these kits fairly cheaply, and the parts can be used as templates if you want to venture into scratch building using card stock.
If you want to learn how to do scenery - extremely well, google a guy called "Luke Towan" - he is brilliant and his videos and very good, clear and detailed and cover pretty much every thing.
I mostly agree with your points, although I will point out there are probably two "wrong ways to do it." One of those is what DJ/Ken calls "building a bad tunnel," vs building a good tunnel. The bad way is burrowing into the face a mountain which sticks out. The good way is to cut the tunnel such that you cut your way into the face of whatever hill or mountain you plan to tunnel through, and then when the earthmoving or boring gets too difficult or costly you burrow under, rather than build a cut through. Thus the need for needing a tunnel in the first place.
To put it another way, don't build something like "Ayers Rock (Australia--look it up if you don't know it and you will find it), and then tunnel through from one end to the other it when you could just go around it. That would be a "bad/unbelievable" tunnel.
DJ/Ken can expound further on this if he wants to.
===============
The second way is: Drainage. It's always...ALWAYS about building scenery for drainage. Rains fall down from the sky, snow falls down too, and all the water needs to go somewhere. So you will always want to at least think about, if not incorporate ditches, culverts, bridges, brooks, creeks...rivers...under any of your lower spots.
If you put in a dirt road under a bridge, plan for at least a small roadside ditch, even it only on one side of your road. If your track crosses a watercourse, you will probably want at least a culvert...or else the water won't have anywhere to drain to. Water and gravity can dictate a lot about how you plan your scenery. It always runs downhill....
IMO, even someone who hasn't thought about something as boring as...let's face it...drainage...will still miss it if it just isn't there. Something inside your head will appreciate the scenery...but they'll still think something is just "off." They may not even know why, because drainage is something everyone takes for granted without looking for it at all.
Unless you automatically just look for it like I do. Maybe I need a new hobby....
