Since you mention fans it seems that you expect this supply will get pretty hot? A wooden case is hardly suitable as it can easily catch fire. A metal case is best, but earth it if it has hazardous voltages connected to it.
For fans, it is best to have a grille and fan so that air is drawn through the grille by the fan and the airflow passes over the hot components.
My first question due to Electrical noise, i mean i want to ask that which is the best for reducing electrical noise? and which will minimize the EM radiation ?? A Metalic casing OR A wooden casing??
A well earthed metal casing will be the best to reduce electrical and emf noise.
For a fan you can use very well a secondhand 12 volts dc fan from an old computer powersupply.
These are generally very quiet and the speed can easily be controlled by means of a series resistor.
I use one in my 12 volts 6 Amp battery charger at home which i built from scratch.
Really it depends on the project, a wooden case is more suitable for a small UV exposure box because it's easy to work with and a metal case is more suited to an amplifier because it provides screening.