Looking for advice - How to prep/smooth walls for paint

[The Drawing Bird]

Hey there.  I'd like to poll the audience, as I know there's some pretty well-experienced people on these forums. 

My fiancé and I bought our first house (a fixer-upper), and we're doing a portion of the renovation ourselves.  There are two rooms that had old wallpaper/flooring/ceilings, and our plan was to paint them and install new flooring.  We're halfway through the wallpaper removal process, and it's becoming clear that we can't just paint on the surface that was under the wallpaper.  There are what look like two different kinds of drywall that have decently large spaces, there is bare wood, and there is bare concrete under various parts of the wall.

My question is: how do I prep these various surfaces to receive a coat of paint?  We're comfortable tackling medium-skill tasks, and we're looking for a respectably smooth wall surface that won't fall apart on us in the next couple of years.  From my initial research, it looks like I'd want to seal the surfaces and add a layer of plaster, like in this tutorial (https://www.youtube.com/watch?v=b1V-g6_FOjM). Here are pictures for reference. 

Oh, and the ceiling has water damage from a leak that has been repaired.  Sand and repaint, or replace the entire ceiling?  I'd appreciate any thoughts you have before we commit!

[The Drawing Bird]

Here's some more pictures for reference.

[SunnyDays]

It looks like the gaps between the drywall are too big to just tape and fill.  If that's the case, then you could cut out some drywall to make the gap big enough to patch it with another piece of drywall, then tape and fill.  I'm not sure what you mean about the bare concrete - is it behind the drywall or showing at the same level in places?  If this is an exterior wall, it should have a vapor barrier over the cement.  If it doesn't, it might be better just to rip out the drywall and add it.  For the ceiling, It looks like you could just use filler and paint.

[The Drawing Bird]

It looks like the gaps between the drywall are too big to just tape and fill.  If that's the case, then you could cut out some drywall to make the gap big enough to patch it with another piece of drywall, then tape and fill.  I'm not sure what you mean about the bare concrete - is it behind the drywall or showing at the same level in places?  If this is an exterior wall, it should have a vapor barrier over the cement.  If it doesn't, it might be better just to rip out the drywall and add it.  For the ceiling, It looks like you could just use filler and paint.

In the first picture, the material that's behind the wallpaper is much more hard and light in color.  I assumed it's concrete, but perhaps it's another type of backer board...

[The Drawing Bird]

I decided to seal the exposed areas, mud and tape the seams with a quick-drying compound, and then plaster over everything to give a uniform finish.  Here are progress shots (of the same area as the last picture on my previous post).

So far, it's turned out a little nicer than I expected.  We'll see how it dries.