As far as decoders are concerned, these come in a large range of sizes, connectors, power handling and function outputs. But they all have one thing in common, one pair of wires on the decoder (the red and black ones) are connected to the pickups from the track, and another pair of wires (the orange and grey) are connected to the motor. The motor must be completely isolated from the pickups or you will destroy the decoder as soon as you turn on the power, be it DC or DCC, to the loco.
May I just clarify one point please? The red and black wires will connect the controller to the track. The orange and grey wires I'm assuming are the standard colours for wiring within the locomotive? Sorry if I'm being dense... I am also in the position of learning DCC from scratch having come back to modelling after many years and so much has changed!
Thanks for writing such a comprehensive description in the first place I have found it very helpful...
Hugh