Audi heating/cooling system problems...

I have an Audi A4 Quattro turbo model
from 2002, and I've been having a lot of
problems with the heating/cooling system.
One problem I have is when I first turn on
my car, before it gets hot, the radiator fan
turns on instantly, and after a while it turns
off, then once it heats up again it turns on
again. It's been a draw from the deck,
every time I use the car i wonder if it's
gona over heat... There was a leak
because of a cracked dorman water outlet
I replaced it but it still seems like the
system isn't functioning like it should... The
heater only blows out room temp air and
the bottom radiator hose/pipe is cold even
after the top one gets hot should I replace
the water pump and thermostat or just one
or the other. I heard it could just be the
thermostat.