The device's state shouldn't affect them from being able to port the number, although I'll admit I've never had it done before.

I would definitely do a factory reset before you sell it though.

 

I would actually flash the factory image if you're running a custom rom to take it back to complete stock and then relock the bootloader. As for the sim, cut and discard it and sell the device without it.

 

I've done this a few times now. Porting your number is a carrier deal, not hardware related however I would wait to wipe your device until you are 100% functional on the new phone. Only reason I say this is there is a point in the middle of the switch where you will get calls and texts to your Gnex even after you've called and activated your new device and started the number port. It should be a brief window but for the sake of continuous service, keep that in mind.

After you're fully up on the N5, do the factory reset and then sell the old phone