If the loco is HO scale, stay away from the DCS system. That system will only work with MTH loco’s.
A good starter system such as the Digitrax Zephyr, or the NCE powercab will work great with your shiny new loco and will be compatible with any other DCC loco on the market.
No, it’s one or the other. There’s no reason to have the DCS system. The MTH loco’s will run most of the functions on a DCC layout. There are a few things that aren’t compatible but nothing that will stop you from running. This includes all sounds etc.
As I understand it the DCS system has a DCC pass through mode, where you can connect a DCC system and control the layout that way. However, it is my understanding that this is an either-or situation. When in pass through mode the DCC system is in complete control and DCS is disabled. Thus you can not run both at simultaneously. You might want to check with MTH to see if you are going to be able to get the functionality you want with the DCS?