Maintenance can be expensive, but it is a proven powertrain.
8.5 quarts of Mobil 1 per oil change. The oil filter is ~$18.00 (it is a cartridge/fleece type).
12 spark plugs.

It is a hand-built engine, so expect parts/pieces to be fairly expensive.
Most of these cars and the SLK32/C32s seem pretty reliable.