Kingdom Hearts The Story So Far for PlayStation 4

So im reading this, and there's a lot of 1.5 and 2.5 and other things, but is it the same as the original kingdom hearts 1 and 2? Whats up with the .5

Is it two discs or one?

Is this literally [1.5 + 2.5] and 2.8 Prologue just in a combo packaging?

I want to start playing kingdom hearts. should i buy this version to start?