hi bro! first of all welcome to TAI
starting with HU, it depends on you, what are the features you want in HU? Like BT,USB,AUX in etc. Pioneer HUs are best for there SQ and features.
Rear speakers fits best on rear parcel tray, if you don't want, you have to fit on boot door or rear doors or in boot inside boxes. if you didn't fit on parcel tray, there will be chances of rattling or you will have to live with less boot space, recommended place is parcel tray.
6''x9'' 3 way coaxial speakers are very good for rear. recom. pioneer,JBL
if you want heavy bass like BOOM BOOM type then amp is required otherwise not.
so tell me what features you want in HU and type of music (bass)