Why people use flex shaft? Because with a flex shaft you can put prop parallel to water surface plus align it in some degree range to get everything prop and engine can manage plus there is no joiner that can breake or cause vibrationns. I did my flex shaft of a 1mm calibrated stainless steel vire, you can not beat this in terms of weight! and price! works brilliant too. if you use straight shaft your prop will be facing water with a angle, that will lead boat end to come off the water more than needed (most props also have some lifting force however) and you will have problems going fast.
so...
going cheap can make your boat fast, but you must think in every spot that is crusial for speed, etc.
and don't save on props, go to ebay there is some cheap sellers..