I'm fortunate enough to be in charge of maintenance for a rather substantial number of German automobiles. These vehicle are stored under cover in ambient temperature conditions year round without any supplemental battery chargers or "conditioners." The average battery life I am experiencing is close to ten (10) years with an all time best of seventeen (17) years from a Sears 36 that simply kept going and going. Contrary to internet learning, I attribute these batteries' longevity to two factors: 1) Each vehicle has a battery cut-off switch which eliminates all parasitic drain from the battery when the car is not in use; and 2) No battery conditioner is or has ever been employed. With regard to the latter point, it is my most definite belief that the use of a conditioner actually shortens battery life by constantly changing and adjusting the natural ratios that exist within a wet cell battery. It is far better to allow a battery to achieve its natural equilibrium than to continually force it toward unnaturally high charge levels.

With regard to the questions posed by the OP relative to his 280 SE 4.5 (there's one of those in my realm of responsibility as well), those who suggest bringing the battery up to snuff with a charger prior to use are correct in that it will lessen the amount of work to be performed by the alternator. If a battery is a little "soft" and just misses starting the engine, there would be no harm in employing a jump starter and allowing the alternator to continue the work. If, however, a battery is way down on its power, the safest action is to use a standard charger overnight so as to return everything to normal. and not stress the alternator and its 40+ year old wiring with tasks that may be too strenuous. So far as battery size, although a 49 may properly fit the tray and appear proper under the hood. I see no reason to try and track down one of these or to pay the premium prices commanded by them. The 280 SE is not an overly large consumer of electricity and the 4.5 engine does not require that much to start. I have found a Group 24 battery to be sufficient for that application..