Yes you can fit another radio in place of the standard one, the Din (hole) size is the same. The connections are really easy as they use ISO as standard( unplug 1, plug in the new 1). you'll find that there will be a plug on the standard unit that you don't need with an after market 1, just tape out of the way.
As for the blanking plate, try Halfords, they have a selection and may be able 2 order you 1.
If you fit a Sony head unit and still want to use the controls on the s/wheel, Halfords may be able to order an adapter as-well
Hope this helps