Generally, one would start with the selection of a proper ADC for the application. Infact, for ADCs like AD7730 (with in-built AC excitation, and PGA, Diff amp, etc.) the best would be to interface teh load-cell directly to the ADC.

However, in my application, the load-cells are supposed to terminate at the Summing Amplifier box. This will sum / average the 3x (or 4x) load cell outputs, condition it, amplify it (0-5 or 0-10V) and then send this output to the uC board having the AT89c55WD controller and a ADC. So hence i have to be very careful in component selection, placement and routing INITIALLY for the Summing Amplifier board, since thats the "first point-of-contact" for the load-cells.

Any error Introduced (or not filtered / corrected) by this summing amplifier will be seen by the ADC on the uC board.

Hence, i am trying to tackle the Summing amplifier board first, then will move on to ADC selection for the uC board.