First the floor then the interior door or vice versa?

OK. Sounds reasonable. I'll do it that way. Then I just have to lay the floor properly under the doors. That can be planned.

I would also take the doors as they are at first and upgrade them to solid chipboard for the rooms in question for an additional charge (I assume such are not standard).

Because if I had taken the doors from the developer, then I would have had doors first, then the floor to be laid myself.

The sequence is always a makeshift solution that EL flooring uses. It is more difficult to lay under the frame. It is better to do floors first, then the frame.

And does anyone have experience with acoustic interior doors?

Who offers something like that?

So I love our solid core doors ... you can't hear anything through them despite the overflow thing in the frame, I had unnecessarily worried ... I actually hear more through the brick than through this door

We have a soundproof door with a rubber strip underneath for my home office. It's great...