before you hand it back put the old speakers back in he he. plus No cutting holes in the back shelf like back in the day..
a sub woofer is different that you will need an amp for it deffo and it’s own power source.. or it will drain the channel outputs to much as will to big and powerful speakers if the actual channel output is low.. most new cars are pretty good sound output wise imo..
If you was changing the head unit that would be a different matter as most these days are fitted or part of the infotainment system and you’d have to buy a different panel to fit it..
If you damage any trim though thats down to you and maybe you’ll be liable for any issues with that if door card need removing etc..