I've been told not to reinstall things like WhatsApp files music photos videos Samsung notes...like everything. I need stuff for work though. Also will I be able to get all that stuff back OK? Do I back up to am external hard drive?
You can get them back but I strongly advise setting new passwords on all online accounts you accessed from the affected device. You should be able to do Google and WhatsApp from a PC, as for the Samsung Account presumably it's possible too.