Entry level speakers are designed to run nicely from the headunit, no amplifier. However, an amplifier will help.
But higher quality speakers will benefit from an amplifier more. For example, if you chose Focal Polyglass speakers, you would definitely want an amplifier.
Speakers vary a lot. So it's important to audition them first, in an ICE store, to ensure you like their sound. Many people do not like Focal! In particular, the treble/tweeter can be a little too loud/pronounced, particularly with their cheaper speakers; however, Performance series may be different now.
Focal SOLID amplifiers are very good.
Alpine make very good headunits, but relatively expensive. You can also have a look at Pioneer and JVC.