French door question.

[Rojomateus]

I currently have a window that looks like this:

So the window sits back in the wall. I am installing a french door here and these are the specs:

My question is. The current window sits back in the wall so I am presuming the door will as well........

There is no mention of a cill in the spec...... I am presuming that one is supplied and that the door sits on this .... but not sure how far the cill will extend..... will it cover the "reveal" depth? Would this be normal? Should I ask?

Also the window company sent out their surveyor to take the measurements (I was having difficulty securing a builder at this time.). I now have a friend who is a very experienced joiner along with his labouring friend who is going to cut out the wall and make ready. But for my own piece of mind I have some questions about what exactly needs done.

The gap that is left by removing the window and brickwork should be in the region of 10 mm to 15 mm wider than the measurement above and should it be the same for the height or more to allow for the cill?

I understand that any gap between outer brick/block work and inner will be covered at the sides by a cavity cover..... but at the bottom where the cill will go is there need for anything such as a damp proof membrane or does the cill act as this.

Thanks for any answers or links to anything on how to prepare the "hole". I have watched various youtube videos but they seem to jump over this part a little...... not so much the sides and I have seen videos re cavity closers but my concern is more the base where the door will sit. thanks