If you are trying to get some cash out of your E70 I would recommend you stay away from replacing it with a 4.6iS. One that is in the kind of shape that will allow you to motor for a couple years without major investment will run you as much as you'll sell the E70 for and one that is cheap will catch up to the cost of a mint one in maintenance and repairs very quickly.

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Again Daily Driven SC'd 4.6iS, whats the point of having a sweet ride if you dont use it all the time.