How do you decorate your place, what kind of style, colors, and feelings? That will greatly influence the type of door you might want. Also, think of your budget, and whether you want to install it yourself or hire a professional (you don't need to tell us, just keep that in mind). Then look around Houzz, in the products page, they have lots of interior sliding doors and hinged ones, from elegant frosted glass in a dark wood frame, to rustic reclaimed wood planks. You may not want to purchase that exact model, but it might give you a better idea of what would fulfill your airflow needs without sacrificing style.

Looks like saltpetre which is leaching through the brick. You may hae waterproofed the outside but what about water leaching from the roof. Water will find a way.... Here is an interesting link... http://www.arcane-industries.co.uk/l1/p281-dampness-saltpetre-condensation.html

It sounds as if the bricks were never properly sealed. Using a penetrating brick sealer should solve the problem.

Ananya, Wooden ACP cladding with glass and natural stone would look good on this. The circles could be covered in external rectangular boxes.