Hy. Even if I am not a mint user, I like the distro. And I like your plans. So I have some sugestion.
The list would be long, but the best is :
Look at the folowing distros: Tinyme and Tinyflux. Even if they are based on pclinuxos and rpm, you should learn a lot. And a mini-cd is 200 mb not 350. I have just buyed one.
You should use openbox + lxde. and here is a small list of apps wich I sugest:
-Opera, not firefox. Opera is smaller and has more features. But if you really want to install firefox add firefdtp and wyzo(torrent) add-ons.
-BitTorrent -Wyso addon from firefox or transmission
-irc-opera has allready an irc client
-GIMP -too big and too many dependencies. Better mtpaint or gpaint
-Exaile! or audacious
-mintApps (except mintMenu, but its still installed)
-beryl-no, too big, too many dependencies.
I wish you succes.