The response is instantaneous. You apply pressure with your right foot and the sleek sedan moves off with authority. As the tachometer needle climbs, it's increasingly obvious that the car under you wants to get on with it. There's a restless urgency here that's rare in a luxury car. That's the result of a 4.2-liter aluminum V8 with twin overhead camshafts and 32 valves, an engine that's made the latest addition to the Mercedes-Benz E-Class--the E420--one of the most delightful combinations of performance and luxury in its class. BMW's 540i used to have sole possession of the honor. Now it can be viewed as a tie ballgame. There's more here than the new engine, of course. And, where it counts the most, less. When the E420 rolled into showrooms in April, its $49,900 pricetag was $2600 less than the previous model. Adjusted for additional new standard equipment, including door-mounted side airbags, that represents a 9.7 percent price reduction.