impedance of an sine wave circuit
The circuit is a very poor wien bridge oscillator and will not be stable - amplitude adjustments will need to be made and distortion is quite likely to be high and vary with time. There are better sinewave oscillators that can be found so I urge you to do some google image research on this. This is the sort of circuit I'd be considering for a decent sine wave oscillator: -



The basic Wien bridge oscillator has nothing to define or control the sinewave amplitude whereas this circuit uses a JFET to do that. Here is a google image page for sine wave oscillators.

As for the output impedance of the op-amp, because negative feedback is used you can assume the output impedance is less than an ohm when driving loads that a 741 is capable of. This won't give you much power into a transformer so don't expect miracles. The 741 isn't capable of delivering (relatively cleanly) any more than a few handful of tens of mA.

If you want to model this circuit I have no hesitation in suggesting LTSpice (free version) but I think you'll find you'll need a transistor buffer between op-amp and transformer if you expect to drive powers more than a few tens of milli watts.