I had the exact same problem about 3 weeks ago. I graduated to D and had a now what.
I got the vette and Ford GT due to a 1 of race I thought was a constant thing (So i basically got 3 cars and a track to realise I can't use them). I'm not a huge fan of the Vette, I find it hard to race. Very fast and no room for error.
I find the Skip barber hard to drive. You need to be on the gas through corners, Very smooth with your turning and breaking. Though once you get the skill (or at least more skill then I) it's quite a lot of fun.
So here I am, at the same stage as you wondering where do I go from here? I was thinking about the mustang (though the cost of all the tracks is crazy)