My advice:
Use RAW and then WB isn't so important (if you get the time then yes, select Custom WB for each setting)
Keep the ISO low to keep noise low
Use a tripod where possible to allow lower shutter speeds
Probably use lens wide open (but dependant on DOF required) - in any case focus on eyes (or eye closest) if shot is head and shoulders
Definitely use available light but be prepared to supplement with fill-in flash or additional lighting
I'm tempted to say beg, borrow, steal (well, maybe not steal!) a prime lens - it'll give you that extra sharpness and more stops
Enjoy it!
