Hiya. My partner has just bought a Polo 2010 1.4 SE, and she has given me the responsibility of modernizing it the same way I have with my own Polo 9n3 2009. So I've been doing my fair share of research.
Off the bat does your son's car have what is called an 'MDI' port? Its found in the glovebox, in the top right hand side. These are handy as you can buy adapters for them so you can plug a modern smartphone into them and use it as a USB socket, or buy Bluetooth dongles for it to go wireless.
If (like my partner's car) it doesn't, you can buy adapters for the aux port to make it Bluetooth if your son is really craving that wireless life.
That's all assuming you want to keep the factory stereo, if not there's some pretty good bluetooth + DAB stereos out there that are designed specifically for the polo of that era that blend in well with the dash. I believe lots of them are compatible with all the parking sensors if the car has them etc.
As for the speakers, a new headunit/stereo improves the sound of the factory speakers quite a lot. They 9/10 come with more customisable EQ settings to tune it to your liking rather than just the basic "bass-treble" settings on the factory unit.
If however, your son is like me (a bit of an audiophile), I really like the ALPINE S range of component speakers as a solid step up from factory. It's what I put in my 9n3 polo myself. Personal favourite website for all of that gear is 'Dynamic Sounds' as they really help with finding the right adapters etc for new stereos and speakers.
Can't say much about the rest of your points, except the window switches. Take the door panel off the driver's door and glue that baby up! That's coming from a botch-jobber extraordinaire though
