I want to replace my radio in my Golf Mk5 GT 170

I would like some advice please.

1. I want to buy a radio that has bluetooth and a USB port.
2. My standard radio can be controlled from the steering wheel, dont want to lose that function.
3. What is Canbus? Does my car have it? or is it an add on when I buy a new radio.
4. Will all DIN 2 radios fit.? The radios in Halfords appear to be completely rectangular but mine has a rounded facia.
5. Had a look on ebay and for about 200 can get a compatible unit. Will they go straight in?

Can somebody share their pain to avoid me suffering?