(Second hand) Xfactory X6?
I've been told it's very easy to get a reasonable pace from the car (easy to set up and drive) - I've also driven them a few times, I was very impressed with their performance.
New conversions are quite pricy but I heard used ones are currently pretty cheap (perhaps due to C4.1, Vega etc. conversions?). It's B4 based: The front end is entirely B4, the rear end is mostly Xfactory. I heard the Xfactory spares are pricy, but as a rookie racer myself I can say I broke most parts on the front end (bad landings and such), so I think it won't be the biggest problem.
My advice overall though: Go to the club you're looking to become a member (or several ones), look what they're driving, and ask who the most experienced driver(s) are. They are guys that can often help you best finding a good setup and increasing your performance - driving the same car as them will help, or perhaps their experience will go further than their own car(s)