Hello all!

I have quite a few questions regarding my Audi A3 1.6 2004.
Now it has 150 000 km and it has started to cause some problems.
Firstly the temperature indicator isn't working..when I drive for couple of minutes it doesn't rise up to 90 degrees as it should, but when I turn off the engine and give the contact back it rises up to 70 and then slowly falls down to 50.
Then the ESP light keeps on shining. Usualy it turns on after driving for 30 km and then it shines till the engine cools down (over night for example) and thus again after another 30 km.
Also now when i start the engine the lights keep on flashing (armature, ceiling light...) and car fan also makes a lot noise after starting the car. Before it never started immediately but after the engine was warm.
I assume it has to do something with the car battery since it's been in the car from its first day. And it's in quite bad condition. One clear indicator is also that when I listen to the radio for 10 minutes (engine off) it drains the battery completely.
Ok enough for now.
Anyone with the same problem or does anyone know how to solve this nerve racking problems !?
Thanks in advance,
giocatore