All sensible questions David. The web link I sent with lofts on, RVwoodcraft, you'll find sputniks on there.
I would put twinwall perspex in the other windows.
You would be advised to put air vents low down inthe front to give you a good air flow.
You can either use v perches inside of which I have some spares you are welcome to or box perches.
I wouldn't worry to much about nest boxes at this moment.
You will also need a feeder, drinker and some grit pots.
Phil