To save money you may get a modern device RBBB kit or maybe a sparkfun arduino mini. They are cheaper and smaller to fit on a mobile platform. But I still suggest you get a regular arduino UNO so you can prototype your project in your home and only assemble the compact cheaper board after your project completes the prototype phase. I can see your project will need an accelerometer and do a running average of the reading and comparing with "level" redings. If the average goes beyond level you reduce the speed of your motor to prevent the rc car from tipping over. Having flashing led is almost trivial too. You can even log your data and determines a max speed for each angle of inclination so you can drive your rc car to max safe speed with the data, how about at? Of course MP3 costs extra money.