I order plugs, wires, etc. from NEEDSWINGS.COM here in the USA. I have no idea what part number, I order from Rob because he knows what is needed.

For my differential, I used Mobile 1 75W90 under advice from the real "tech heads" here on the forum.

Engine oil is any of the 5W40 full synthetics that has "MB229.3" on the side. I would avoid 0W40 unless you are in a cold climate (which you may be).
Full list of approved oils is here:
https://operatingfluids.mercedes-ben...sheet/229.3/de

Power Steering and convertible top uses Pentosin CHF11S.
Brakes use DOT4 here in the USA, I do not know how brake fluid is rated in the EU. This site is MB's information page that might be helpful:
https://operatingfluids.mercedes-benz.com/

Other than the air filter, the engine is an M112 which was used in the SLK320, you can search "2003 MB SLK320" for plugs, wires, and most fluids and be just fine. In fact, that is what I do.

In the USA, the Fuel filter is a Mann WK720. But over there, I would search "MB SLK320 fuel filter", and I would expect the search to yield a BOSCH part. I BELIEVE the BOSCH number is 0 450 915 003 0.