You may as well go all the way down to about 40% of your sample rate. Any higher frequency content much higher than that will just get aliased anyway. If you need to capture transients faster than that, you need to rethink your strategy.
It doesn't make too much of a diff whether the filter is before or after your amplifier.
The output impedance of your 623 is good enough -- you shouldn't need a buffer between it and your ADC