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: 
I would suggest using lower end Atmel AVRs for beginning robotics projects. An AVR can take a lot of abuse and can sink and source more current without burning out than the PIC microcontrollers I have used. 

If you are going to be building multiple projects, you should consider investing in building your own breakout boards. You can design your own PCB with standard power circuitry, a bunch of headers for I/O and whatever microcontroller you choose. You can get your cost down to around $15 per board for the PCB and then a couple bucks worth of components. This way, you can leave your project intact, instead of having to harvest out the microcontroller for your next project. As a bonus, you will gain valuable experience in PCB design and soldering.
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.