I bought the 2009 3.6 last year to enter a couple of races in the VdeV championship. Having had so much fun, we invested with my friends in a 2011 3.8 car to have a chance to do well in the championship, and so far, we are number 1 after three races so we are very happy indeed.

Out last race was the 6h of Motorland Aragon where finished 1st in class and a surprising 3rd scratch behind an RSR and a 458 GT2 so pretty cool stuff.

The Cup car is indeed the most basic racing that Porche offers, based on the 911 GT3, it has exactly the same engine, about 200 kgs lighter, sequential gearbox and semi adjustable suspensions. 

You cannot buy it new from Porsche unless you're gonna compete in the Carrera Cup but now you can buy a second hand car and enter loads of competition like VdeV, 24h series and even ELMS.

Reading from your post, I think you should gauge how much fun you're going to have with your GTS on the track, if you  get hooked, then the natural progression will be to move on to a GT3 type car, and then after to a Cup car. 

Have fun!!!