Installing yourself can save some money and you can just buy the speakers on ebay. It depends on how much you want to spend. With audio equipment you pretty much get what you pay for. Some of the better speakers will be MB Quartz, Focal, or Crystals. Of course you can never go wrong with Infinity or Polk Audio. A five channel amp would probably be the easiest to install for your application. But you might get more power if you buy a seperate amp for the 6 1/2's and the sub. MTX, Rockford Fosgate, JBL is fine if your not an audiophile. If you really want to step up to clarity look at PPI, Phoenix Gold, Macintosh, Memphis. For your sub to be heard at all when your driving then you are going to at least need 500 watts. Look at 12" woofers. JL Audio and Kicker are the brands of choice.
Either way a new system from the ground up is not going to be cheap. Don't forget the wiring. If you buy cheap cables you might as well buy cheap equipment.