Hey I'm purchasing an Infiniti G35 2.5l that has engine issues and I'm planning to swap the engine with a 2007 Infiniti G25 (2.5 vqhr). I'm looking for support to find out what I'd need to do to make the swap work or to find out what solutions are possible.

Would I need to get new engine mounts? Would I need to use the G25 harness in the body?

It depends, if the car you're buying is a 2011-2012 G25, you'll just need to swap the motor. Everything else should remain the same. What you've posted reads as you're swapping the motor with a vq25hr that the car comes with.