I've searched and I've read through the wealth of info on replacing the stock HK system in an R53 (mine is a 2006 MCS), but I remain unclear on these questions:

Can I replace the HU, speakers, and amp, but use the existing wiring?

If the answer is yes to the above, with a full replacement of the components and speakers, can I move to 4 ohm components rather than 2?

Thank you.

 

Yes. After you disconnect all the components, they are just freestanding wires and you can connect the ends of the wires to anything you want.

 

Thanks. And move to all 4 ohm is not an issue for existing wiring?

 

Existing wiring is not an issue, unless you're installing a high power amp (and play LOUD). In that case, you may need thicker gauge wiring for the system to reach its full potential.