Nitrous is not what you are looking for unless you are going to spray everytime you put your foot on the gas.
A turbo will not help low-end power WHATSOEVER.  A turbo must spool up, which results in what is known as turbo lag.  (turbos kick ass on your top end though...).
There aren't many ways to shortcut your way to some low-end torque as it is typically a function of displacement and displacement alone.  For $1,500 I'm betting you could work the intake and exhaust a bit but be careful, it sounds counter-intuitive but upgrading to a much larger diameter exhaust or intake system will actually hurt low-end torque.
In order to produce low end torque engines like to breathe through longer runners on the intake side, and an exhaust system that doesn't just dump after the manifold.  It all has to do with the manner in which the air is flowing (turbulent vs. laminar).  All of these modifications, however, help the car produce a lot of power on the top-end.  At high RPMs the engine doesn't give a fuck about laminar flow, it just wants AIR and as much AIR as it can push through.
In short, there is no efficient way to make that Audi of yours a torquey beast for $1,500.