btw.. coax's are pretty crappy its all bodged into one and not too great, if your on a uber budget then yeah maybe (only consider if your not amping)
if you really dont want the amping costs then the best things to look at will be alpines speaker range thats designed to run from the head unit… see here…
http://www.caraudiodirect.co.uk/alpine-spe-17c2.html
but your standard golf headunit will be sending out crappy signals anyway so at minimum id advice a headunit and front speaker change!
having speakers installed in the rear messes up sound staging and makes it sound confused and whack.