Google/Rockauto a thermostat/water pump for a 2004 MB slk320.
Proper mix is 50/50 distilled water / Zerex G05 undiluted.

As a note, my gauge reads just below middle while at 55-60 MPH with no A/C, which is 186 degrees.
When I idle in traffic with A/C on, I have seen 208 degrees which is slightly above middle on the needle.
Mine quickly cools off when I am up to speed again.
186 without A/C, 193 with A/C
( I have an UltraGauge which displays temperature )