Is Unity what I need? Can Unity do what I need?

I have developed a web app that you can use to develop 3D environments. The app is using Three.js an consists of 3D objects, created with the tool, Sky Cubes, particles to simulate rain, snow etc and sound. Since JavaScript in the browser is a bit limited I now would like to create an app where you can view the place you created and I have started to look att Unity.

So...

Is it possible to via script import 3D objects from a server together with it's materials? The objects are in obj format and can be altered in any way since my web app controls all of that. Three.js can also export in many other formats if that is needed.

Simulate rain, snow etc?

Load sounds from a server?

Load images from a server to use as a skybox, or similar?

If so is this a hard thing to do? I know JavaScript to day and know how to 3D stuff in the browser.

Where should I start :)