Some questions about modeling (with Tabula)

1) My goal is to make a fluid tank with the front face that shows the fluid level, much like the openblocks one. In order to do that, is it correct to basically create an empty cube, with 6 shapes forming it, so that inside it is empty and the fluid can be rendered?

2) Is it possible for some faces (in my case the sides) to have one dimension = 0? The reason would be making texturing easier...

3) About texturing: I've understood the concept between the texture offsets and all, but when it comes to actually making the texture
(I'm using paint.net) It's quite hard to measure all the pixels and stuff! Isn't there an automated way, like a blueprint?