Replacement windows

[Cpd]

So after many years ignoring my dreadful windows my sister offered to buy me new ones as a belated 50th birthday present 😁 I am happy to fit them myself as the detailing inside will be tricky due to different wall / insulation thicknesses. There are only 4 windows all approximately 700x1200mm. I am in an extremely exposed location facing the sea. My house does not have mega insulation…. 70mm pir retrofitted to the inside but no floor yet….. the retrofitted insulation was detailed very well so no gaps and fully sealed. The old windows were split into upper and lower panes but new ones should be be single pane to let more light in. I would like to keep the frames as small as possible so there is as much glass area as possible. I am based on the west coast of Scotland I. Argyll. My questions are. 
1. recommendations for window supply only 

2. anything I should be aware of … pitfalls 

3. type of window I should get 
4. have I missed anything….

thanks for any advice. 

photo is two of the old windows, the other two are the same.