The batteries for Euro cars are a standard "DIN" design.
The battery for a W124 300 is generally a DIN 66.
I.E, around 550CCA.

However CCA's - (Cold Cranking Amps) aren't the be all and end all.

With modern computerized cars and high output audio devices as well as devices such as alarms that suck small amounts of amperes when the vehicle is idle, it's my opinion that a battery with a large amount of residual Ampere hours is also highly desirable.

Optima's are a fine battery but expensive. There is a specific Optima battery made for all model Mercedes Benz's - right back to the Ponton's.

You don't screw around with the wrong polarities either. There are specific AC Delco batteries designed for Euro cars with the positive pole in the right location and the standard clamp down ledge.

Batteries. You get what you pay for. The O.P has an original OEM MB battery that has lasted him 8+ years - excellent performance. Let him come back in a few years and tell us how long his AC Delco lasted for.

Bosch is one OEM supplier for M.B. Varta is another. You can find them at other battery suppliers other than the dealerships.

Batteries are one of those bits of equipment you always do a little bit of walking around to find the best and appropriate unit that you can afford.

The newer the vehicle i.e. those who now contain a raft of electronic computerized equipment in them, the better the power supply system needs to be - and this starts with the battery.

In another twist. It is illegal in Australia to run a vehicle without the battery being tightened down by OEM clamps.