Aftermarket stereo - EASY!! Just get another lower dash cover that has had the pocket cut out and mount the stereo there.
Speakers - Put them into a box which goes on the shelf between the back of the seats and archway. Hidden but still practical.

Pain in the bum bit about the glovebox - If your buy yourself it's a pain to get at, if there are 2 of you, the glovebox lid will hit your knees each time you change station/cd etc.

OR (for a ute)do a small bracket off the seatbelt mount, and put a small box speaker up there.
EVEV make a box that fits behind the armrest (between the seats) mount a forward facing speaker there with the original type one in the dash.

--- Updated ---

All these are removeable/replaceable with the greatest of ease and no damage to your prestige interior.