How do you intend to run this "custom installation".
To me that indicates an independent workable system of some sort installed on a drive.
If this is the case why not use a netboot install or a netinstall and then simply boot to the login (text) and install from there using apt-get or aptitude?
The netboot install is VERY small, netinstall very small.
What you basically get is a kernel (and all that includes), grub, a sources.list and the basic package management system.
Netboot install address';
Stable amd64 http://ftp.debian.org/debian/dists/s...tboot/mini.iso
Stable i386 http://ftp.debian.org/debian/dists/s...tboot/mini.iso
Testing amd64 http://ftp.debian.org/debian/dists/t...tboot/mini.iso
Testing i386 http://ftp.debian.org/debian/dists/t...tboot/mini.iso
This runs at about 21MB download for a bootable disk.
Net install address easy to find on the Debian Site. I always have trouble finding the netboot ones.