How to set the sampling frequency for this filter+
When designing an anti-alias filter for a lowpass signal, you need to know

1) The sampling frequency fs
2) The signal bandwidth fb
3) The quality of the signal you want to achieve

Your passband goes up to fb. It should be flat enough for your application, you may not want it -3dB down at fb, you might want less attenuation. A Chebychev filter has controllable passband ripple.

Your stopband depth must be enough for the quality of the signal. If you want a maximum of 1% alias products, then a -40dB stopband will be sufficient (you could use less at certain frequencies). For 0.1% aliasing, you need -60dB stopband.

For a post-DAC filter, the stopband starts at fs-fb, and goes up to infinity.

For a pre-ADC filter, your stopband edge depends on exactly how you are going to use the data. There are three choices.

If you are going to do analysis only in the specified passband, then your stopband starts at fs-fb. Signals between fs/2 and fs-fb will alias to below fs/2, however they will be above fb, and if you are analysing only to fb, will not cause a problem.

If you want no signals aliased at all, even if they are out of your specified bandwidth, then your stopband should start at fs/2. This might be useful if signal harmonics are present, and they form part of your processing dynamics, for instance detecting zero crossings. Harmonics, when aliased, become non-harmonic, and mess up peaks and zero crossings.

If you want to do as little digital processing as possible, then your stopband should start as close to fb as you can economically make it.