Sounds System in a 2001 Toyota Land-cruiser?

Ok so we've had this car for 10 years and all 6 of the speakers are blown out. The problem is we can only replace them through Toyota and it will end up being about 1500 bucks. I was wondering if instead we could just upgrade the whole radio/sound system to something that would be compatible with other speakers and what the effects of this would be (would it mess up the existing gps/interface?). Any thoughts would be appreciated.