[JamesBryant]

Not quite - what I meant was build it from whatever you need in the simplest way that does what you need. That may be just discrete components, or it may be a handful of ICs, or a mixture. And don't overlook the possibilities of embedding a minimal microcontroller to do any logic - it may be the smallest, cheapest, simplest solution.

And the other half of the message was to build it as a well-defined entity which can be proved by itself before it is used in its final application.