Simple analog anti aliasing filter good enough if signal frequency is far from sampling frequency ? What about equivalent time sampling?
The sampling theorem states that you need at least "two samples per one Hz of bandwidth". Anti-aliasing filters are required when the bandwidth of the original analog signal is not well defined and a finite bandwidth needs to be enforced. 

In your application that doesn't seem to be the case. If all you want to sample is really just an unmodulated steady-state sine wave, the bandwidth or your original signal is already limited. No anti aliasing filter is needed and you can simply sample at any rate slightly larger than twice the sine wave frequency. 22.05 kHz or 24 kHz in your case would work fine.

ETS is a bit tricky since you need some sort of a trigger mechanism and then a variable delay off that trigger. The equivalent sample rate depends on your delay steps and the whole process depends on the signal being periodic with high precision. 

You actually CAN sample at a much lower rate. The theorem requires 2 samples per Hz of bandwidth. For a pures sine wave the actual bandwidth is almost zero so you can sample with a very low sample rate and still don't have any loss of information. You just need to manage the mirror spectra properly, i.e. your application needs to be aware of a frequency offset.