T3 forms a Darlington pair, with T5
T4 compares the reference voltage set by the potentiometer with the output voltage. it's in common base configuration. It turns T2 on, when the reference is exceeded by 0.6V, thus cutting off T3.
Yes, D1 is a zener diode.
BZ1 is a buzzer, which is turned on by T1, when a the current flowing through R1 exceeds a certain threshold, set by R1.