transient simulation oscillator output with hspice

I want to use hspice to do transient simulation with some deterministic noise added to VCO (like sinosuid voltage on VDD) , then do FFT for PSD to evaluating the impact of the noise on phase noise of output oscillation (around 1G).

But I have some confusion,
1. which FFT window should I choose to minimize leakage or so-called picket-fence effect? (remember that non-integer number of period results spectral leakage)
2. how much transient simulation time is efficient for resonable accuracy
3. sampling frequency? (I already know to use fixed time step in transient simulation)
4. other tricks or attention in simulation?

Please give your advice or some reference on this procedure!
Thanks in advance!