Sensor buffer with minimal component count
I'd rather try this very simple current buffer:







simulate this circuit &ndash; Schematic created using CircuitLab

1) Hall sensor will have to sink no more then 1.5 mA, I beleive this is quite within reach.

R1 has be a little lower than R2 so that when Hall sensor is open circuit Q1 base is higher enough than 6V pullup to be sure Q1 is off despite voltage tollerances.
Just make sure not to go beyond max BE reverse voltage (a few volts at least anyway).

If Hall sensor current were too high R1 and R2 could be increased or even removed at all.
This would have to be payed with a lower maximum operation frequency and a somehow lower noise immunity due to higher input impedance 

2) Output will be pulled down to  \$V_\text{CE(sat)}+V_\text{BE(on)}\approx 0.3\,\text{V}+0.7\,\text{V}=1\,\text{V}\$ which is quite below the 2 to 3V you believe enough for reliable working