I've converted the PPM signal from an rx to PWM to directly drive a small motor, there is plenty if open source code to do this.
I would recomend an arduino mini for use in a robot rather than an arduino uno, it is much more rugged and compact but less good for prototyping as it doesn't have the "push in wire connectors" but instead you solder the wires in so it is more for final solutions than prototyping.