I'd say that the guide you gave the URL to is a little incorrect (or outdated ?).
To build modules external to the mainstream kernel, you need the kernel-headers *and* kernel-kbuild packages.
From kernel-kbuild's description:
This package provides kbuild tools for Linux kernel 2.6. It is used together with the kernel-headers packages to build out-of-tree kernel modules.
But the best way (that prevents quite some problems) is to install kernel-source, nvidia-kernel-source and build your own kernel packages using make-kpkg.
Note that if you are using kernel-source (or a vanilla kernel from kernel.org), you need to first configure your kernel (make menuconfig) and *then* install/build the nvidia drivers ('make-kpkg modules' if using debian packages; else launching the binary executable). You cannot properly install the nvidia drivers before having configured the kernel, because some required files are only created during that stage.
EDIT: About the "code model `kernel' not supported in the 32 bit mode", check the following discussion, which seems to match your problem:
(Basically, you are mixing up 32 and 64bit code - you need to cross-compile to solve the problem.)
Hope this helps !