ROLLS-Royce, move over. Mercedes Benz is coming out with a $250.000  limo,  the Maybach, aimed at stealing your customers. The car will have luminescent fenders, reclining rear seats, a humidor, a bar. a pen set, and even a drawer for your slippers. Some of the manufacturer's execs brought a  prototype over from Germany to see what America's CEOs thought. The Benz crew parked it in Coopers Classic Cars & Cigars on West 58th Street and invited potential buyers in one at a time, one per hour, to sit in the back and get comfortable with their favorite  single  malt scotch and a cigar. A psychologist was on hand to record their impressions.   Coopers   owner   Elliot Cuker said, "This car does everything but make love to you."