Basically, it's best to use separate circuits for train power, signals,
and lights, whether controlled directly or with DCC. Never use a single
controller or power supply for the whole layout. (A very large layout
should be divided into "power districts" each with its own supply.)
Add up the amps for the devices in each circuit, and buy or scrounge
power supplies to suit. You'll need 12-16V DC or AC, and anywhere from a
couple of amps on up. Just make sure they're well-protected with
fuses/circuit breakers. Transformers are cheaper than DC power supplies,
and for lights ans signals you don't need DC.
I've used cheapie train set controllers, most of which have both
controlled DC and uncontrolled AC outputs. Controlled DC can be used for
day/night transitions. Typically, such controllers can run 2-4 dozen
grain-of wheat lamps in series-parallel circuits (used to reduce the
voltage to each lamp, thus extending their life).
I look through the wall-warts (plug-in transformers) at thrift shops,
you can find ones with 9 to 12V DC (or AC) and anywhere from 500ma to a
couple of amps output. Ideal for small loads.
You can use common return (earth, ground), just make sure the ground
cable is heavy enough to carry the total current if all devices were
switched on at once.
HTH
Wolf K.