For flat areas, are there loose stone laying around in enough quantity for stone production?
For mountain areas, are there steel and components further inside the rock that can't be seen from the outside edges? (Not that it goes further in from the edge, but that you'd have to dig just to find it?)
1. Maybe? It depends on how much "enough" is. I usually pick flat maps and I haven't had any trouble getting enough stone to build walls for all my buildings and outer defenses, but I don't build huge mega-bases.
2. Yes.
Yes there is enough. You should watch out for a place with just 2 types of ore, as 2-3 might be annoying in color choices and stuff.
In Mountains you can only see the outside layer. If you look closely you see that after the first row of whatever stone the behind is grey, which only changes if an adjacent tile is open space. But still here no fear, with late tech you can find hidden resources in the ground or distant.