Is there a need for buffering in this case?
You need a resolution of 0.4 mV in order to detect your signal. Accuracy is a different question.

The voltage divider created by the source and load impedances introduces a constant gain error factor that can be calibrated out by applying a correction factor later.

As long as your DAQ has at least 16 bits of resolution, you should be good to go. Since your measurement bandwidth is so low, most sources of noise can be filtered out, and since you're not looking for DC signals either, offsets can be removed as well. All of this (1 Hz low-pass, 0.001 Hz high-pass, gain correction) can be done in the digital domain, after the ADC.