i just bought a 2001 audi a6 wich has 226.000km

i bought it knowing that the engine wont start, but the man who owned it had no idea if the motor was stuck or anything.

When i switch on the car there seems to be no problem, all electronics work fine and it does not look like there is low battery.

But when i try to start the car, Nothing happens.
all the lights in the dashboard e.t.c turn off and i hear 1x click sound
( just like there is a bad battery )

engine does not crank at all.

can anyone point out obvious reasons why this could be happening?

i am going to try a new battery.
i read on another forum that i should check out the main 125A fuse.
i also read that it could be auto theft system (but in that case, the engine cranked)