2SC1815 & 2SA1015 are good transistors with 60V VCE. The BC337 & BC327 are only 50V.
The only transistors that handle substantial current are the output transistors and their drivers.
The VAS transistor pair handle 12mA top (Q5) and 6mA bottom (Q7).
The emmiter follower (Q6) driving Q5 handles very little current, but needs to have as higher ft as possible and good at operating at low currents.
The two input transistors only handle 500uA.
Q10 is only a 1mA constant current generator but it needs to be a low capcitance, high frequency type.
The rest of the transistors are not critical and you can use what you want, within reason.
BC549 & BC559 are only 30V VCE which means you would need to reduce the amp supply lines to +-15V
No. The variable resistor sets Iq, the quiescent current (no signal) flowing thru the output transistors, Q1 & Q2. This is adjusted to around 30mA to minimise cross-over distortion. The amp voltage gain is set by resistors R13 & R14: A= 1+ (R14/R13)
Bit of an open-ended question; it depends on the transformer type and the core material. As a rule of thumb, I would say 1,600 sq mm. 1 turn per volt is fairly normal for that area core. An alternative way to judge is by the power rating of the transformer: 100W upwards should be fine
C core is the best:
(1) Lowest size
(2) Lowest stray signals
(3) Lowest output impedance
(4) Lowest weight
Any transformer type will do though, but with the other types be prepared for some work to reduce hum introduced both by inductive and electrostatic coupling.
PS: amp schematic has been updated to issue 2: only minor mods