Greece is 220VAC @ 50Hz
Changing the computer should be quite easy:
1. Move red button on back of power supply from 110 to 220.
2. Obtain IEC power cord with proper plug for Greece. The end that plugs into the computer is the same as the 120VAC power cord.
The montor may just need the same power cord. Look on the back of the monitor for the nameplate. It will show the voltage range, mostly likely 100-240VAC @ 50/60Hz
Look at power brick for speakers. You may need a different one or an adapter. If speakers are powered from monitor (some newer ones are), nothing else to do.