If your engine is in a good state of tune and the battery isn't old and roached, you shouldn't really need much extra capacity. Fix the battery drain issue first. If the engine is healthy, it should start easily after a pre-glow and 1-2 compression hits. If you're having to crank for extended time, you should be looking to find out why. Are you certain the alternator is charging the battery fully? When I had my 300SDL, it was getting moved around the shop with an old Honda Civic battery (<400CCA) and that was adequate. The H8 battery that the US cars use will let you crank for a *LONG* time (minutes) before it runs down to the point it no longer turns the engine.