There are two packages that maintain a local cache of debian packages with
very little manual maintenance: apt-proxy and apt-cacher. Apt-proxy is
available only in woody and apt-cacher is available only in sarge/sid.
I am currently using apt-cacher in sarge and it is working very nicely.
But I have a rather more mature/expensive system than you. I have three
Linux boxes and apt-cacher is running on one of them. It allows me to
install the same packages on all machines with only one download.
It does cache the packages that are installed on the same machine that
it is running on, but it does not allow you to redo the netinstall from
cached packages. This is because netinstall will (I think) overwrite
the installed apt-cacher software during the initial phases of net install.
You may be able to install apt-cacher as one of the first packages that
you install, prior to pulling in X-windows, etc. Then, apt-cacher would
make cached copies of almost everything optional in your install. I
recommend that you use apt-cacher and install it as early as you can, so
that it will cache copies of as much as possible.
On the other hand, Sarge is not yet stable, so you may be downloading
updated copies of a lot of stuff anyway, because you want the latest
versions.
Other considerations: apt-cacher needs apache server to serve the local
repository that it generates. This may be more than your hardware can
handle.