I'm looking for some sort of platform that allows me to almost immediatly render 2D, has built-in GUI, sound engine, and if possible, network code. On top of all this, LUA-based scripting would be a must. This would allow me to rapidly prototype simple game concepts, because I still can't code games using my engine, far too complex for this... I hear people talking about SDL a lot, so perhaps I should check that out...

SDL stands for Simple Direct-media Layer. It's by no means a Rapid Development architecture for games.

I don't know of any offhand that fit your description, but I do know that there are lua bindings for SDL, so prototyping your game in LUA might be an option.