Hi, AP1.
I ended up having to add way too much power and suspension work to the SLK. Plus, you're going to win another one. I found that by the time I added enough parts to take the series, it ended up costing almost as much as the CLK would have.
If I can make a suggestion, I'd sell the SLK and buy a CLK. That should only need T2 tires and a Sport suspension to beat the other Benzes.
If you can't afford that, I would say go with Turbo 1, Racing Transmission, and at least Sport suspension if not Race. Run T3 tires or so.