A cheap uPVC window fails at the gasket, the roller, or the weld — not in the brochure. Ask for these before you pay.
Profile and weld. Corners should be cleanly fused. Thin recycled profiles flex and leak in the first heavy rain.
Operation. Sliding, casement, or tilt-and-turn should match how you use the room. Bedrooms often need casement or tilt-and-turn for ventilation; balconies usually need sliders.
Mesh and grill. Plan mosquito mesh and safety grill in the same frame. Adding them later usually looks messy and leaks air.
Hardware. Multi-point locks and branded rollers cost a little more and save a service call after monsoon dust.