Given the size of your layout, I don't think you need to even think about power districts. You don't have a large enough layout. Power districts are used in DCC on large layouts (usually club layouts) where a short (eg running through an open switch) will bring the whole layout down. With power districts, a short in one district will only bring that district down and leave the rest of the layout running.
Your main concern at this stage should be focused on the wiring - principally, the wiring for your track power. If your layout is currently wired for DC, you can use the exisisting wiring.
If your layout is under construction, you have an excellent opportunity to put in a track power buss that can carry the full DCC load. The track power buss is simply two wires run the length of your layout (or at least underneath or close to your main clusters of tracks). Most DCC books suggest using 16AWG wire. Most discussion forums suggest using the thicker 14AWG and even 12AWG wire. The decision is yours. Don't go less than 16 AWG.
One type of wire is trailer hookup wire. This is stranded colour coded (in a variety of colours - red, white, black, blue, yellow, brown) wire that is usually available in 25', 50' and 100' rolls at a cost of about $0.10 per foot. It is also available in several wire guages from 18 AWG right up to 10 AWG (the lower the AWG number, the thicker the wire). Pick two colours (eg white and red) and use these two colours throughout your layout. You run these two wires from your command station underneath the layout where it follows the largest concentration of trackage. At the end, simply put some electrical tape over each end.
From here, it's a simple matter of dropping track feeds through the top of your layout down to the track power buss and soldering the track feeds into the track power buss. To simplify your life, as you install the track power buss, solder in some pigtail wires about every 3 - 5 feet. Click on this link to see what pigtails look like and how to do some wiring.
http://www.railwaybob.com/Modules/TrackBuss/TrackBuss01.html
The key in DCC is to solder every connection. Don't use mechanical joints like the guillotine suitcase connectors. And to keep things nice and neat, use some shrink tubing over the solder joints.
Bob M.