Supposedly, you already have an Elise. That is a damn good track car. What level are you? Have many track days do you have? Last thing you want is to learn in a GT3 RS..
That being said, I can kinda tell you don't have too much track experience. If you want to track your car, you have to consider the related expenses. A car like R8 will be expensive.. Why? Because it is heavy, it has a big engine. So it needs big brakes, big tires. And it will use them fast. Engine oil will be like 9quarts, every couple thousand miles... Track insurance goes up really fast for expensive cars... It adds up really quick.
If you want to enjoy the track, consider cars like E30/36/46 M3, Lotus Elise/Exige, GT3/RS. All used of course.
Even better, you can check forums like SCCA for already built race/track car sales: