I'm fairly new at this as well, but I'm happy to share what I know.
I went with battery power right out of the gate, so I can't speak to DC power needs.
What locomotives you have and what systems they already include or don't include will affect the cost and complexity of the battery conversion.
I have an older RailBoss system from G Scale Graphics in a Bachmann 55 ton Shay, along with a Phoenix sound board. I really like how it works, but it is a bit cumbersome dragging a full-size R/C transmitter out to run it.
I just finished installing an AirWire system in a Bachmann Climax. This system utilizes the built in DCC decoder and sound board. So far I really like the system, it has a ton of customization and control features so the learning curve is a bit steep. Someone with a background in DCC would have an easier time, but I came into it cold.
I also have a live steam locomotive that will be getting a radio control system from https://www.rcs-rc.com/
These systems are primarily for Live Steam locomotives, but on board battery power options are available too.
I don't have direct experience with the system yet, but I can say Mr. Walsham has had excellent customer service answering my many questions.