I couldn't tick one or the other of the boxes – I think the choice has to be made contingent on so many variables. In fact we have painted timber, double-glazed bifolds (with a fabulous retractable pleated fly-screen) at the end of the dining room leading out onto a terrace and a painted timber, double-glazed slider (with a timber-framed sliding fly-screen) on the other side of the kitchen (but on the same axis) opening onto a raised timber deck. Each works really well in terms of function and aesthetics but they would not be interchangeable as the different locations determined which was more suitable – but they have been made by the same highly skilled window craftsman and have given us nothing but pleasure. And on a hot, steamy evening with both open we have the most wonderful cross ventilation.

Wow I too have the same glass above my doors. Mine are all internal swing doors therefore I don't have your delema. I'm in the middle of renovations and mine are staying. My walls will be dark and modern with all the trim white with the original character and it looks great. With your wall, it's a lot to deal with therefore would suggest replacing the slider with a glass door with timber trim, keep your walls light modern and bright and paint your trim dark (any colour you favour) here is my example to give you an idea of what I'm talking about hopefully you can see the value here is a snippet of my renovation good luck....hope my idea gives you plenty of ideas

Could always have some custom made if you wanted just two opening doors. Not a cheap option but worthwhile looking into initially. May not be as expensive once the whole job is added up. Many companies make 'standard' size french door unit which are around 2100mm high x 1700mm wide. These would obviously fit but a builder would need to rework the surrounding surfaces. Another option is to look at the french doors with glass sidelights. These are really just double entry doors with sidelights. Have a chat with some manufacturers first then a chat with a local carpenter/builder.

What performance is more applicable for your climate? Double glazing will perform significantly better than single glazed whatever in regards to heat loss. You can't beat the air gap factor if you're trying to keep avoid heat loss. Yes there are "Low E" single glazed products such as viridian Comfort Plus which depending upon which side of the glass the absorption coating is placed it will perform in different ways, but bear in mind that since it also will reduce heat gain in winter it can actually work against passive solar building performance if not properly applied. In other words, it's not a "cheap" replacement for double glazing. In Victoria, we generally spec standard double glazing and use external controls such as blinds to mitigate heat gain issues in summer. Thermal Performance Specialists such as an Energy Raters understand the technical nitty gritty around this issue the best, and always good to seek their expert advice when applicable.