re: external repos
I think its your script and do what you want! haha my feed back would be not to add them by default have a "add 3rd party untrusted" like ubuntu distro's do and might even have a back out command as well (undo) so that if people do a apt update all it wont go insane on them.
I personally never update my browser in BT as I only use it for hacking things, not a daily distro. it is just there to support the plugins that I use and mostly debug web applications. run it all in vmware so if shit hits the fan I keep a snapshot about every month. more important for me is the stuff you got in there already
the coolest thing I think you could do is figure out how to make your script check a source to see if a current SVN release will cause a problem or crash. w3af is always one I just update then like a week later I need to use it where I dont have internet or something and find that my current build was wonky ..that sucks. I like to keep up on msfw but it seems like every time I actually need a working copy I get burnt... never when lab work. haha
also might add a auto update function