To find which cars you need to do each event, look at this: http://i.imgur.com/LNTN0.pngfeel free too add me, not really touched MP yet, but might give it a go tonight.
Im finding Single player really difficult! What car should I stick with to go through the upgrades (nitros etc)?
I used it and it makes it super easy to do! Mark off which race you do as you do it.
As for cars, Gallardo is my weapon of choice along with the R8. If you can get the R8 set up right, you can do every race possible with it.
Oh, and for Needle Point, I used the Konny rather than the Veyron when people start to struggle there