You'll need an adapter/fascia to fit in the head unit to the dash, this will cover the blanks spots. These are about £10 from ebay and Halfords etc
There's also a stereo wiring adapter you can use. This will go between the new stereos wiring and the cars wiring and should keep the the standard sub/amp in the boot working. Also available from ebay. Autoleads do these, usually available from Halfords or any ICE dealer for about £10-£15.
Ebay is likely to be the cheapest option. Dont think there will be too much difference in brands/quality.