This is where the "computer" should be replaced by the "appliance". Simple raspberry machines in rugged enclosures running ONE piece of simple software off flash cards. If it breaks it's a matter of either just inserting the backup flash card (because the old one died) or worse case replacing the raspberry inside the box. Either way it's dirt cheap.
The shop can easily be given 3 raspberries and a dozen flashcards when the system is first installed, for almost no money. Even if the software vendor is out of business when it breaks in 10 years, they are good to go in no time at all. Any auto mechanic can swap the raspberry inside the case in 10 minutes and be up and running again.