The 4.6 engine came from Alpina, not ACS, so I'm guessing this is a 4.6is with ACS wheels. Have you got a link to it ?

Changing from standard radio to nav is a massive PITA, and the parts are not cheap either. Not worth it IMO. Better to spend an extra couple of grand buying one with the spec you want.

I'd like a 4.6is, they seem to hold their value better than the 4.4i so the gap to trade up is gradually getting bigger