Okay,
1) Well, most devices use /CS to disconnect data bus but not address bus.
2) There is parameter called "Input capacitance" which may cause problems if device (here: MCU) cannot provide enough peak current during destination is switching on. It is sensetive for pFET-outputs of C51.
3) yes, buffers increase current, but not only; they protect MCU from destroy if a bus problem occures from accidental reason.

As for me so I have used 244 and 245 for most my projects which have data/address bus expanded.

Regards,
Oleg