Just to give you an idea I had the same model but an automatic. When I bought it it had 115k miles on it and I paid $3k for it. This was also 9 years ago.
That code is a catalyst efficiency below threshold code because the header you just installed has no cat. You can either get it fixed from a tune (which you should get any way) or try spark plug defoulers.
Someone on another forum put the zdx throttle body on the stock IM with a 3.5inch intake and said he notice a dramatic increase in low end torque and throttle response even before a retune.