Some questions compiling linux kernel with make-kpkg

[Akronix]

Hello, I'm doing my first steps compiling the linux kernel and adding a new system call. I've managed to do it, but I still have some question.

For this task, I has set up a Virtual Machine guesting debian 7 64 bits. I am following a tutorial found in Internet of how to do this (I'm not able to post links yet...)
And as documentation, overall, a manual of how to create a custom linux kernel in Debian GNU/Linux and the manual page for make-kpkg.

So far, I am using the next script to compile:

#!/bin/sh
make-kpkg clean
KBUILD_VERBOSE=1 INSTALL_MOD_STRIP=1 CONCURRENCY_LEVEL=2 time make-kpkg --initrd --append-to-version=-$1 kernel_image

And well, I have the following questions:

1. Right now, I'm defining kernel_image only as target. Should I include also kernel_headers? when should I compile and install the kernel_headers? what are they for?
2. If I use as target: binary_arch, is exactly the same than using kernel_image and kernel_headers?
3. I put INSTALL_MOD_STRIP=1 because I read that it compiles it faster and the output is much smaller. I know it reduces the number of modules being compiled. how much is this reduction? Should I use this flag for a final compilation for a final use of the kernel in my computer??

Thank you in advance!