I would agree with dxw00d, large base and moving pretty slow. If you really don't want to put a line on the floor then i would go with some IR distance sensors or ultrasonic. I would also get a kit of some kind, defiantly with the rotary encoders and a large base with motors you will easily be able to control with the arduino. Is the robot going to have to go from say a tile or laminate floor to carpet? i think a bump like that might unsettle your valuable cargo. Stick to it man! cool project.