Vortex Sound Research

I am looking to buy a small Casio style keyboard for music composition. I wake up in the middle of the night with new music in my mind. I want to start putting them down on staff paper but I want the ability to record them quickly and go back to sleep. I'm completely inexperienced in this category of synth/keyboard. What would you recommend if I want to get something on the cheap side? A sequence is not likely going to be enough to record my ideas.