Question: I am looking for a starting point for my project, preferably using popular systems (ones there is a lot of support for). I have an Arduino Uno, a Raspberry Pi, and a lot of willpower :) Anyone here built a project using the systems above?

Observation: I'd like to start with a simple line-following vehicle and build up afterwards.

Answer: 
It really depends on the project.

For a line follower robot ( in your case ), using the Atmel's AVR series is the best choice. Specially ATMEGA16 or even ATMEGA32. 

Because the line-follower is a small project and the Arduino is too much for it. And the other advantage of Atmega16 is that it is cheap. If it is broken or faulty then you can change it easily. and it also provides PWM for controlling the speed of motors. 

So with atmega series you can build the most powerful line-follower . No need to spend money on getting prebuilt boards and ...
If you want to go further than using microcontrollers you could run ROS on you [Raspberry Pi](http://www.ros.org/wiki/ROSberryPi/Setting%20up%20ROS%20on%20RaspberryPi).

I have build several robots and a couple of other projects using a [RoboCard](http://robocard.dk/) (site in Danish, but can [be translated](http://translate.google.com/translate?js=y&prev=_t&hl=en&ie=UTF-8&layout=1&eotf=1&u=http://robocard.dk/pages/home.php&sl=da&tl=en)). The RoboCard is build around an ATMega, so that certainly is a viable route.