It's easily worth it, it'll allow you to do certain B-Spec races much quicker and will also allow you to do the FGT championship when you are the required level. I believe you win the GT by Citroen Race Car from the Dream Car Championship B-Spec so don't buy it.
just win the x1 at level 30 challenge get all bronze or better if you can and there are three different x1's for grabs its the most insane car in game to drive. unless you dont have a wheel then its too insane
Saving your money would be the best think to do. Once you have more than enough Credits to by a high-end Race Car in the used car dealership, or the FGT if you can, then you should try to buy it