It'll be quite difficult to get a flat response down to zero 0Hz
also boards tend to have SMPS noise and other rubblish there
1 MHz is reasonable start, but since you have Fs= 100 MHz, perhaps 10-40.
Remember to prevent alias responses, with a 40 MHz highest freq, your aliasing filter will need to be 100dB down or whatever at 60 MHz. and stay down ... since it will alias to hundreds of MHz.
So for that reason, to make your aliasing filter easier, choose the lowest downconversion freq you can get a nice response.
perhaps 5 to 35. So now the alias filter only needs to be 100dB down at 65 MHz. that's easier.
Also, why not just undersample ? Have you considered that? Many good converters are still pretty good in the 2nd nyq zone, through dynamic performance does degrade.
Sample clock phase noise will be proportionally more critical....
I would consider a direct undersample. Once you introduce mixers, there are more spurious responses.....
Also, watch out for the 2f converter response. IE harmonics produced in the converter..... IE 2 x 32 MHz = 64 MHz == (100-64)= 36 MHz