Jim,
Any DCC system will work. And you need not change wiring if your wiring works with DCS or DC, it will work with DCC. Pretty much the same standards for gauge etc apply.
As for systems, I would recommend one of the Big 3, Digitrax, North Coast Engineering (NCE) or Lenz. The only thing to consider is the amperage needed. I recommend the higher powered system from which ever you consider. Digitrax has an 8 Amp rated version and NCE has a 10 Amp version, and I don't know what Lenz has.
There are not DCC switch motors per se or any other components. However, if you want to use the DCC system to control the switches, you need to use stall motors to through the points such as Tortoise.
the best advice on chosing a system is to chose one that someone you know - a friend hopefully - already has. Or perhaps one that a club near y ou has. That way you have someone local that can answer some basic questions as you get started. I found I had some really simple questions to start with but once I got started it all came together very well.
As for decoders, I am familar with large scale sound decoders, but I think Soundtraxx has some now . I use NCE D408 SR decoders for motor control and on a few locos have installed separate small sound decoders. Most of my locos do not have sound. My control system is Digitrax. I also am using some Digitrax and Lenz decoders. but for large locos the NCE D408sr can be beat. It has a rating of 4 amps continuous and 10 amps stall.
If you have studied the HO descriptions for DCC, the same apply to O - it is 2 rail just higher amperage.
Another note, you mention a number of European models. If you are in Europe, digitrax makes a European version of its radio control system. I don't know if NCE does and as of last check by me, Lenz did not offer RAdio control
By the way, the radio control is all throttle/receiver based. It is just the way signal gets sent to the command station. The command station will be the same regardless. With radio you just connect a receiver into your throttle buss.
