UK R170s do not have rear speakers as standard. Only the Bose upgrade. It depends on what you want out of a system. the R170 front mounts (with a bit of help from Dynamat and some decent speakers) are capable of some really good sounds, but not the lowest octave of bass. It is possible to put a couple of subs in the back behind the seats (there are the Bose fittings / space), which is what I intend to do. But the sound I am getting out of the fronts is so good, I am not sure I can be bothered.
Front speakers are not too difficult to change. Just be sure to put loads of Dynamat around the bass driver. It helps to stop vibration, but more important is to keep sound waves generated from the rear of the speaker from cancelling out that generated from the front of the speaker.
For speakers I would recommend the Focal 165 V30. These work well with just a head unit, and give decent bass. A steal for £170!