i really love to buy 1 but the price 

Got many positive review such as improve handling and stability. 

RM1k for a pair? That is not far off from flying in a set of Swift metric springs myself which are lighter and typically half to 1 coil less for the same spring length. Not to mention much more stable spring rate across the stroke.

Don't opt for lowering spring if you have bodykit, you will just ruin in across the road humps. If you do not change to the proper lowering dampers, it will most likely mess with the handling as well. With the spring most likely coming lose once the damper tops out.
It depends on whether you drive in heavy traffic or not. Not a fun thing when you get stuck behind some arsewipes driving insensibly in front of you.
Why would a spoiler be a cause of the issue? Except for the car standing still, rain should not be hitting the rear windscreen much.

Anyways, can always invest in a bottle of this. Lasts for a good 6 months and acts the same as rainX but lasts longer and is an extra layer to prevent scratches when the wiper sweeps across especially with the rear windscreen constantly being stuck with dirt kicked up from the turbulent air.
http://osren.com/product/coatz/all/04-fluorine-glass-coating