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: 
The Arduino is really an AVR Atmega328p. The Arduino is a fine off-the-shelf implementation of this microcontroller, but if you make many of them, you can buy the chip for less than $3 each in bulk, and it requires very little circuitry to run on its own -- a crystal and a couple of capacitors to run at 20 Mhz, or not even that if you can run at the built-in 8 MHz resonant oscillator frequency.

It's fine for generating control signals: Servo PWM, step/direction, control for H-bridges, etc.)

It's also OK for running sensors: Ultrasonic time measurement, IR voltage conversion measurement, on/off contactors, etc -- this includes whatever optical sensor you'd use for "line sensing."

There will be a little code space left over after doing these tasks, so the simple control loop of "is the line to the right, left, or center of me -> do the appropriate turn" can be built into that system. However, as soon as you want to do something bigger, like path planning, environmental awareness, memory, SLAM, etc, you will not be able to fit that into the Arduino.

Thus, the best system for your requirements probably includes tying all the physical hardware to the Arduino, and then talking to the Arduino from the Raspberry Pi. The RPi has a modicum of CPU power (700 MHz ARM) and RAM (256-512 MB RAM) and thus can run higher-level control algorithms like path planning, localization, SLAM, etc.

If you go with a bare AVR controller, there are UART outputs on the Raspberry Pi, but the problem is that the RPi is 3.3V and the Arduino Uno is 5V. Either go with a 3.3V Arduino version, or use a voltage divider to step down 5.0V output from the Arduino to the 3.3V input of the Raspberry Pi. I use a 2.2 kOhm high and 3.3 kOhm low resistor and it works fine. You can feed the 3V output from the Raspberry Pi directly into the RXD of the AVR, because it will treat anything at 1.2V or up as "high."
I built a line following robot with an Arduino before. It was really simple to do and all we used were color sensors on the bottom inputted in the Arduino, and then of course some motors for the wheels. 

But using an Arduino allowed us to have plenty of room for other components we wanted to add on to make our robot do more things. 

Also, if you want to see some line following code we used just ask in a comment, but it obviously depends on your setup with the sensors and how you want it to turn at intersections and things like that.