Quake3World

I am making a complete modification of Quake 3. New weapons, new models, new singleplayer maps with secrets and puzzles. Am i allowed to sell this when im done?

Yes, provided you use the GPL'ed engine or ioquake3 - in which case you more or less have a standalone game. Any changes to the source code will need to be made available again under the GPL.

... and a reminder that all the game assets (textures, models, menu items) are still under copyright, so as long as you aren't distributing any of that, you should be fine.