Your concept looks fine.
You'll have to experiment with the heater to make sure you've got enough heat. Remember that a certain amount of energy gives you a certain heat rise, so make sure you are getting enough heat to work in your coldest-expected environment.
The single relay should work, but check the coil current (or resistance) to make sure you're not exceeding the Arduin's 40ma rating.
With the Thermistor, you'll need to make a voltage divider (another resistor in series). Again, you'll probably need to experiment to determine the best value for the other resistor.
If you connect the output of the voltage divider to an analog input, you can use that to regulate the voltage. I
If you connect a potiometer to another analog input, you can compare the two readings and use the pot-setting to vary the target temperature. (One end of the pot goes to 5V, the other end to ground, and the center to the analog input. The valuse isn't critical... Anything between 10K and 100K should be fine.)
...I don't need a fast heating device, but only a stable one.
The stability comes from your thermistor & feedback system. Like any heating system, you turn it off when it gets too hot, and on when it gets too cold. There is usually some hysterisis (the turn-off temperature is slightly higher than the turn-on temperature) so that the system doesn't oscillate rapidly and so your relay doesn't "chatter".