Disclaimer:
I don't have any MTH engines... yet.
But having said that, I believe that with MTH, "how easy it is" depends on which particular engine you're interested.
Once you have the particular locomotive chosen, you should try to download the MTH manual for it, which may describe the light board inside and what kind of connections are offered.
I've notice for at least one of their locos I like (HO GP35), the internals of the "dcc ready" version may be different than the one with proprietary DCS control.
The dcc-ready board has a connector for a standard dcc decoder, plus there may be a jumper or two to reconfigure (the manual tells you what to do).
Once configured, the engine should behave as would any other dcc-ready engine with an added decoder.