Variance in ECG lead resistance ; its effect ; CMRR
Considering the load is > 1M and source is 1k min, adding a tolerance of 50 Ohms has null effect considering 5% of source is good and 50 parts per million compared to load is effectively null.  I might consider &lt; 100 ohms OK.

The bigger error source is contact pressure or motion producing galvanic skin voltage from the change in pressure and change in capacitance between electrode and skin.  Thus you want pads that maintain relatively constant adhesion pressure.