So that means I need to purchase both a Legacy CAB-1L and Command Base -1L. Are these Lionel items? Will I be able to run my 3-rail MTH locomotive in DCS mode while operating the Lionel crane car with the CAB-1L system?
Yes to all of these.
The minimum spec of a CAB-1L/Base-1L is because although the command crane is a Trainmaster Command Control (TMCC) device, the TMCC controller and base (i.e. CAB-1/Base-1) are no longer produced, and have been superseded by Legacy, which builds on its predecessor's feature set.
The CAB 1L/Base 1/L package is an 'economy' verson of Legacy--it can do a little more than a CAB-1 (so far as Legacy locomotives are concerned), but not as much as the full-blown Legacy remote/base. If you already have the full Legacy system, CAB-1L remotes allow you to add additional basic controllers to your setup at a lower price point.
DCS and Legacy/TMCC signals can run on the same track without interfering with one another. I've actually run DCS and TMCC diesels coupled together using a DCS remote (I had a TMCC base for some time before acquiring a CAB-1 some time before the 1L version was released). It was a real juggling act, but I did it without making a mess
---PCJ