Hi Jason,
The modulator frequency of both ADS1018 and ADS1118 is 250kHz (as shown in Fig.40) and is independent of the output data rate. Please note that the oscillator frequency, from which the modulator frequency is derived, in this device is not trimmed. It can easily vary by a few percent. Thus the modulator frequency and output data rate will vary by the same amount.
I would also take the ADS1120 into account if they need higher input impedance when using a voltage divider to measure the +/-10 or 24V signals.
Regards,