Advice 1: UW value

It is an important parameter informing about the heat transfer (loss) of the window, i.e. the amount of heat that escapes 1m2 of the window per hour when the difference on both sides (interior, exterior) equals 1K (1°C). Many factors contribute to that value, but still it is the most important information defining the window.

Example:
Window model 1 has Uw = 1,6W/m²K,  
Window model 2 has Uw= 1,3W/m²K,
Which parameter is better?
Of course Uw= 1,3W/m²K!

Passive windows reach value below 0,8 W/m2K. However, if you are not building a low-energy house, but just look for some windows to your flat, you can consider windows with Uw of around 1,0 - 1,3 W/m²K.