To begin with, DC and DCC are fundamentally incompatible. DC is Direct Current. DCC is Alternating Current. If you put an ocilloscope across the track, DC i a flat line where DCC is a classic sine wave. Those systems that suggest you can run a DC locomotive on a DCC circuit do so by breaking the AC into chunks, and only feeding current to the DC locomotive on the "uphill" side of the sine curve. This is very hard on the motor, and something I would caution against.
Reliable DCC operation requires very good trackwork, and very good conductivity, which means lots of feeders and clean track. This sounds onerous, but it also has great advantages. It is not necessary to split the layout into blocks with insulators. It can all be one big circuit. You park a locomotive with the decoder, not by shutting the power off to the piece of track the locomotive is sitting on. You don't have to worry about reversing loops, because the decoder will "decode" which half of the AC it needs to maintain motion. There is an extra level of complexity in learning how to use DCC, an additional cost in equipping all your locomotives with decoders, and your trackwowrk has to be solid, but overall the wiring of the layout is very much simpler.
With DC, you have to insulate, isolate, and power every separate block of track. You need a battery of DP/DT switches to turn power off and on to each block. You need a diagram to figure out which switch works what. Some will say that's where the fun is, figuring out why your locomotive stalls or arcs on a switch, but I'm not one of them.
If you wire your layout for DC operation, when the time comes that you make the change to DCC, you'll end up ripping out all the DP/DT switches and tying the wires into two bundles, + and -. All your insulators will be redundant, in fact will inhibit conductivity, and should properly be replaced with metal connectors for more reliable operation.
When I find the time, my old DC layout is on it's way out the door. I have to put decoders in twenty-odd locomotives, but my new layout will be DCC from Day One (Digitrax Zephyr).
If you're just starting out, I'd urge you to choose DCC, which is the way of the future, and wire your layout accordingly. Go buy a used laptop and incorporate it into your system. Install block detectors, program it so that your locomotives will stop if there's an open switch leading to an oncoming train. Use it to set your signals to green or red, depending on other traffic. Use it to drop crossing gates. DCC adds so much to the realism and enjoyment, it's hard to summarize in a couple of paragraphs.
Do some reading. I spent a year researching DCC and various systems before I chose to go that route. There are several DCC forums on Yahoo and elsewhere with many knowledgeable people. Check them out.
In any case, to answer your specific question, "What do I need to do where the power is reversed?", you have to ensure that the direction of DC current in the loop matches the direction of the incoming train. Then, while the locomotive is in the loop, you have to change the polarity of the DC current in the outgoing track with a DP/DT switch. (DP/DT = double pull / double throw — get em at Radio Shack, or use Atlas switches).
HTH
Cheers
Scott Fraser
Calgary, Alberta