You can download individual directories using the svn tools like this (for example) :
svn co http://subversion.assembla.com/svn/chdk/trunk/core
Of course, it won't do nocaC_230 any good unless he just wants some light reading
You should be able to build with just one platform (+generic, libs, include, tools etc) in place.
There is no such thing as a "mini" CHDK, but many of the features mentioned are in modules, and so they don't have much impact at run time you don't use them. There are some things that can theoretically be turned off at compile time, although I wouldn't bet that all of those defines actually work any more.