For a small starter layout running scale speeds appropriate to tight curves and short runs you don’t need 18 volts on the rails. I’ve set the adjustable boosters on my layout to 16 v and it is rare that we come close to running at the max DCC speed step on a multi track main line with 30 car freights and 12 car passenger trains. Typically we run the freights at about 40 scale mph - around 2/3 throttle for most of my locomotives. IMO the trains run and look better at more modest scale speeds. The AtlasO switcher ran beautifully with 12 volts DCC on the rails. No question - if you like high speed running you need more that 12 volts.
With DCC one issue is estimating how much current capacity you need for full operations on a large layout. That depends on factors such as the current draw of the locomotives you run, the number of locomotives running at the same time, train length, grades, and other power demands like smoke and passenger car lights. The good news is that today’s 2 rail models are generally less current hogs than in earlier times - and we have a lot more choices in control systems to operate them.
