An analogue buffer, as you seem to be describing, would be completely application dependent. There are many factors at play....source drive capabilities, coupling methodology, isolation, signal envelope delay, bandwidth, voltage levels etc etc.

So as you can see there is no real answer to your question unless you focus in on some specific design. Such focus has to look at required performance, chip selections, component specifications, ideal versus practical tradeoffs, cost issues, power supply requirements among others.

By the way, a signal holding circuit, if needed is not a simple buffer. A sample and hold circuit uses a very high impedance device to replicate the voltage of some signal level stored in say a capacitor.

Michael Karas