If you don't want to go mad money wise like me I got a decent HU, Pioneer F900bt for £700, then new speakers front and rear, JL amp situated underneath the passenger seat cubby and using the existing Bose sub which I may replace in time but is good enough for now. So apart from the HU you wouldn't know anything else wasn't standard until you listen to it
All in all it's cost me about £1,500 including professional fitting but I got a very good deal on the speakers as they are 2nd hand