Questions concerning Unity, Skyboxes, Sketchup, Google Earth, and Snow & Lighting.

Okay before I begin, although it may be obvious, I am completely new to this. I've only been using this program for a few days now, and I'm running into a few crossroads with how I should go about using it properly, now..First question

In unity, I'm attempting to make a city, with inside flooring (Upstairs/downstairs,interior design and a real inside/outside feel to it). Now I understand that Unity is not a program for modeling 3D things, So I have downloaded Sketchup. Thing is, It's looking like I will need to hand make every single building inside and out. My question is, What are my options that make this process easier? If I wanted to create a city, Identical to one found in google earth, Would I need to create each building one by one? Are there any programs that mass produce cities? with interior design? I'm not in a rush or anything but on the scale of large city, It would take awhile to finish if i did this by hand.

Second Question, Skyboxes...I'm looking for a step by step tutorial to making one in very good resolution, Using Photoshop or Gimp. I've searched around for tutorials, but I couldn't find something to explain it to me properly. I want to create one so that I can add it to my first person camera, or world background using that Render settings method.

Third Question, Snow and Lighting, ...First how can I make snow accumulate over time? Like build up on the floor? and surroundings? What would I need to learn in order to do that? also, Snow build up to where if the player steps in it, it's like in his foot steps, the crunching of the snow and the thickness it creates, ( I know that is a very broad question, and concerns multiple things )...I'm just trying to figure out what I would need to learn in order to create that in my game. Also ...lighting...I'm looking to create light from my world inside unity so that buildings within a city can create shadow....

If you've read all that, just wanna say thanks for starters. and if you have any suggestions or ideas that can help me, I would very much appreciate them.