Originally Posted by X5SND

If you're entertaining this idea because you don't want to part with your e53 I understand; because in that case the cost is a bit easier to justify. However, I'd definitely advise against this route if you're simply after better fuel economy and an updated engine...as its going to cost much more than your X is worth --- in which case you'd be further ahead to dump your X, and move into a model that meets your needs.