Start with the HU, you will need loom adapter, steering control adapter, fascia and maybe line boosters as the bose amp runs very low.
If you want to go the whole hog you can replace all the speakers and sub but you will need amps too.... Can get expensive
Just replacing the HU makes a huge difference, only downside is loosing the oem BT if you have it but almost all decent HUs now have BT built in anyway
Have a look in the I.C.E. section to see what me and others have fitted