Adjusting offset (and possibly gain) of ADC preamp OPAMP with DAC
Bruno, Digital Pot DS1804 is very easy to control and has 100 ohm taps. Digital pot CAT5114VI-10-GT3 is bit harder to control as it is controlled via 3 wire serial interface, it has 32 ohm taps. Using DS1804 will get you the gains you need. Using zener diodes to limit voltages in you case is fine. 
Good luck.