To the first question, it depends if you want a 3.5" disk or a 2.5" disk. A 3.5 is cheaper because it's bigger but requires you to plug it into the power grid (may not hold true for all, but I think it does). If you want it to be mobile you gotta get a more expensive 2.5", also has less storage.
Whatever one you need, Western Digital makes good external drives. A 3.5" disk is the WD Elements which is about 100 euro for 500 GB and the 2.5" WD Passports are about 90 for 160 GB. So it really makes a rather large difference, but get an Elements disk if you only want to use it at home or another fixed location.