u mentioned incamera flash too bright as a reason... but if i'm not mistaken u can actually compensate the in camera flash with ur camera. or u can stuff something infront of the flash to weaken power. and if u also understand ETTL, it might also be a factor with overpowered flash.
then there is iso etc etc coming into play.
may i suggest that u just play with ur incamera flash and understand how it works, then consider an external flash?