Aluminium Clad Wooden Windows
When I read this report on the embodied energy and life cycle of various types of windows, it looked like aluminium clad windows (and doors) are the way to go.
I’ve since found and researched GRP windows and the cost, lifespan etc. means they are what I’m going for. This is heavily influenced by the sea (salt air) location and being about 2/3rds the price of Alu clad wooden windows & doors.
Level thresholds are a serious risk to airtightness. Most use brushes (often with a thin plastic strip in the middle) to ensure air tightness, and I would have severe doubt about the quality of the seal and long term airtightness.
I have come to the conclusion that a semi-flush threshold is probably the minimum acceptable, although I’ve been looking at folding/sliding doors rather than just sliding, so the situation might be different. Be careful to check the quantity and type of the seals.