I would go with the ga16 airbox, since it's cheap and the intake pipe is way bigger than the Micra one (like 70mm diameter) which is also bigger than the throttle body (45mm)
I would recommend the redneck way of doing things so that is K&N and yes second hand K&N Also have a look at my blog, that is a efficient and correctly dimensioned and correctly installed (valve cover tube) redneck filter. I can ask the designer on what physics or other formula he had based it on but it works like he predicted.