Okay, ran into my first issue. I checked out the Debian Med svn repository, got the latest BioCocoa source code put into a tarball, and am now trying the svn-buildpackage command. It gives me an error:
dpkg-checkbuilddeps: Unmet build dependencies: libgnustep-gui-dev (>= 0.12.0)
Insufficient Build-Deps, stop!
The only version I can get from apt-get is 0.11.0. I guess my Debian version is too old, I downloaded the latest one I could find at debian.org, Debian Etch (debian-40r4a-i386-netinst). Or there is some sort of "developer" system I should be using?