I've always had a doubt.

Do you paint first the pieces and then glue them, or you glue the model and when assembled you paint it?
Also, is it possibly to clean the paint from the models (specially from bad painted ones), or would it leave a bloody mess which would be worse?

With the muhreens I usually completely assemble, pose them and glue them down with super-glue. This way I can plop out any pieces that happen to be in the way and just re-glue them back on when I'm finished. With bigger/more complex models I keep this system to a certain degree, but sometimes leave some stuff out and paint it apart from the rest of the main model.

Yes you can, at least to a certain extent. If the models are made out of metal just dip them in any solvent for a couple of days. Plastic on the other hand is quite finicky, so people tend to use instant green, iirc.

It depends on the models, but I usually paint my guard after I assemble them fully, since there's not much details I have to get too.
Now, for my vehicles, I've left certain parts unglued, to allow for easier painting.

And there should be ways to remove paint easily from plastic, I know I've heard of some cleaner or something that does it well but I forget what it was.

Oh cool, so I guess it doesn't matter that I suck on anything related with painting, colors and such.

On metals not at all. Plastics can get a bit finicky in that regard, since whatever corrodes paint also corrodes plastic.

Try to get GW spray paint, because I can't find any other types in my area that don't kill the plastic.
For paint, there are plenty of modeling paints, but remember to always thin your paints.
I can show you examples of models that are painted with minimal effort, if you like.

Please, show me the examples of your people.

A few of the more notorious examples.

THEY ARE MELTING!

I'm really not good at painting, but this is my best effort so far. He still needs some extreme highlighting and a proper base, but eh, I'm lazy.

That's pretty good actually. The hammer could use some work though.

And basing isn't really necessary, but it does give your army a nice feel to it, as well as sort of bringing it together visually.

It looks very good, but that tube of the hammer makes me think that you trapped Nyan Cat.

It made me think of ketchup relish and mustard.

Considering the 6th edition is out, would it be worth to buy Assault on Black Reach now, or should I wait until they release a new starter kit, if there is ever going to be another one?

Unless you want Orks or Space Marines, wait for the new set.