Hopefully some of the European members will chime in about value restore vs. unrestored. You have an extra option with this if it's identical on all 4 sides (excepting which is the door and where on the inside the phone would be) in that you can restore two sides and let the other two be, and just turn it to suit your needs...
If it's for you, do what makes you happy.
If it were mine I would fully restore it, and keep it somewhere when people, especially young people of a certain age, who have never and will never be in a phone booth, can go in and make a call--cooler still if you can hook a land line phone to it. Great I think for the phone in the rec room--er--man cave...
If you are looking at value/usage you might also talk to props companies that provide props for Hollywood to see what they look for in a piece to rent out. I am guessing restored but with a used look to it, not all pretty and shiny.
Nicole