Hi Kestrel, glad to see you're interested!
Learning curve is... well... that's something I really don't know how to answer. I played exclusively on the middle difficulty level until the League got properly underway, at which point the highest (Professional level) kicked in. The jump from mid to high wasn't too bad, I felt. I never played F1 challenge, so I can't compare. But I think that joining up and playing with all of us will prove very helpful to sim racing overall. There's a good range of talent here, with some players showing considerable experience and knowledge. I count myself behind the curve, rather than atop it, so I feel I'm learning a great deal just by being around them... observing, listening, and patiently working my way up against myself first. I have no illusions about being a fearsome competitor against our top drivers, but I do feel that I'm able to participate and race along with them and have a blast doing it.
As for Starforce... no issues that I'm aware of.
Your specs looks quite capable of keeping up. You'll want to optimize your graphics and such offline to be sure you've got good framerates. Best test is to fill a track with a dozen AI cars and see how your PC handles everything. Online should be improved over offline simply because your computer won't have to process all the AI opponents/physics, just your car's physics (plus rendering other players' cars, but that's strictly graphics).
Joining is straightforward. The server is listed in the lobby. Just Join and enter the PW when asked, and you're all set. You'll be prompted to pick your car before you enter the server completely.
Any more questions, or you want anything better explained, please fire away.
Hope to see you soon.
Cheers.