We would advise it is completely dependant on the space and your requirements. Walk-in wardrobes look great if they are kept tidy, however a plus for having doors is that it can conceal any mess and give more protection for clothing. Another trend for walk-in wardrobes is to have smoked glass doors mixed with open elements this could also work with matt lacquered doors. Alternatively you could choose to have open elements for items such as bags, shoes or coats (things you know that are easy to keep tidy) then hinged doors for other modules.

You need to consider the space into which your doors open to. Ideally opening out of the new extension. If its not too spacious, or might cause things to be crammed, only then can you go for bifold. Bifold will really only come in handy if you have a problem with space. Do you have pictures or a plan you can share?

The thermal qualities of new ones would be preferable. When I had new patio doors (sliding) I couldn’t believe how much cosier the room is now compared to the old 1980s one. Your examples are beautiful. I recently had one of the large double glazed panes changed to accommodate a cat flap (safety glass cannot be cut) and I was quoted £370 - £650 for one pane. I chose the cheaper quote but just to let you know of possible costs to change the glass.

I think the height is irrelevant because every door is different. So I think you have to buy a long spindle and place the knob or handle in the centre of where the cross bar would be (if it were a solid door rather than hollow) This picture of an old door shows the traditional placement