Think any speaker etc that you put in will be an improvement... to drive them properly you would still have to get something more than the standard head unit to drive them. And that is the one bit you can't change.
You could buy the wiring loom adaptor from newministuff and then add your own amp / speakers..
...but buy gear from Ian would be easiest way (fitting aside)...and you would know that it all fits etc. I know newministuff adapt the rear tweeters on the 6x9's they fit so that they fit in the standard grill...might find your alpines dont fit without something being adapted. He also adds sound deadening, and re-wires everything...hard to duplicate without knowing what you are doing.
....or just go the whole hog and drive to liverpool and get it all upgraded in a day...by someone that knows how to take apart the car and get it back together. Mine has less rattles than before it was done.