What kind of supply do you intend to use - a linear or switching one? If linear, then it must has a very good smoothing stage and be with capacitors 470nf/1kV in parallel to each of the rectifying diodes together with a choke. That's because of the ringing effect when it's used in such applications as players, tranceivers, amps and other devices which require perfect oscillogram of the signal from the supply. Voltage might be higher than 12V, up to 15 for example, the player has its own stabilizer inside. The voltage in the car grid is 14V or even a few more (14.4V), so don't worry if your supply doesn't give exatly 12V. In fact voltages up to 18V are applicable.
The output power of the supply depends on the power, consumted by the load, its power is on its label. There must be also a 20% reserve, because using a power supply on 100% is a risкy business.