can you buy any type of external HDD and it'll work with any mac or do you have to buy specific ones that cost like €999?
You can purchase any that the Mac has the correct type of port for(usually USB or FireWire).
You can buy any disk with FireWire, Thunderbolt or USB. If you've got a 2010 or early MacBook Pro, buy a FireWire disk, or if you've a newer computer, a Thunderbolt disk. They'll be faster than the USB, but you can buy an USB hard disk
Any drive should work, but you may need to reformat it. Most drives tend to ship pre-formatted for windows (NTFS) so you may need to use disc utility to reformat it.
Retrieving data ...