2004 Vito stereo and speaker replacement.

I've just bought a 2004 Vito 115CDi Auto Van and I'd like to change the stereo and speakers if possible.
If I change the head unit, is there an interface available so I can still
use the steering wheel controls and dashboard display unit?
What size speakers do I need to perform a straight swap? And are the speakers easily accessible?
I'm really pleased with everything about this van (especially the performance) except the stereo, it is truly awful, and this is the upgraded CD player. I can't imagine how bad the standard one is.
(I'm in the UK)
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