Looks very smart as a combination, I've not seen Caspian Blue with Midir before, night be worth calling your local dealer to see if they have something similar.
Stuff that might help on the forum... rlfaxe posted some caspian blue pics here, to get an ideas of what the colour looks like in the flesh
Your last question I have the D2 ES so have the basic stereo, I'd say it fine for a car... better than my previous BMW 1 series, on a par with the BMW 5 series that I had, but not as good as the Audi A4 Avant I had with the Harman Kardon subwoofer in the boot.
My perspective when it comes to car stereos (I know many on the forum wouldn't agree) is the car is pretty quiet but there is always going to be an element of engine/road/wind noise which undermine the purity of the sound. I was driving a couple of weeks ago trying to work out what the rushing sound I could hear over the stereo and aircon was, it turned out to be the road noise of the Peugeot 308 about 3 cars in front of me, it's those kind of things you are never going to eradicate.
Im concerned the color might be a bit to dark to show off its lines. But for some reason the other alloy choices on the model I selected just dont look the part. And the Midir's go better with very dark colors or white (which is a no go for me) :S!
The fact youve gone with the V40 with that history speaks volumes! On that 'note', I think sound should be ok, its one of the things I forgot to check on the test drive.
I havent seen any real life photos of the black midirs actually. Maybe it's a sign!