There is no such thing as platinum core. The two basic design architectures are copper core - nickel electrode, and copper core - platinum electrode. Some platinum electrode plugs are considered "high performance" and others are "long life". The later typically are "double platinum" with a platinum button welded to the ground electrode.
Mercs from your era typically use non-resistor plugs, and resistor plugs might not perform as well. Most manufacturers do not offer proper non-resistor plugs for Mercs, but Bosch does, and they are usually not too hard to find.
Consult your owner's manual and use the recommended Bosch Super plugs for your engine. These are copper core nickel electrode type. They should have a list price of no more than $2.50 each and should nominally last for 30K miles.
My 2.6 calls for the Bosch H9DC or H9DC0. The OEM plug was the Bosch H9DC0. The "0" suffix indicates a heavy duty electrode. They are harder to find and cost more.