The BMS normally handles overvoltage, undervoltage, overcurrent and short circuit in addition to balancing. Check out the specs to see that the one you buy actually does all that.
36V is the nominal voltage, which is the voltage when the battery is about half full. You ned 42V to get it completely full, and the low voltage cutoff is at maybe 27 volts. Li-ion has a pretty big voltage range from full to empty, and wear out rapidly if taken outside the limits (even being near the limits causes wear, but some wear is deemed acceptable in order to get good capacity).