You ask how much money you'd have to save so you'd be ready by your 60s… but how old are you now?
The 1959 Cadillac Miller Meteor, which was the year and production model used for the Ecto-1, Ecto-1A and Ecto-1 at Universal Studios is not one of the more frequently produced variations as far as I'm aware (someone who is more of a Cadillac expert correct me on this if it needs correcting) and as such they retail for a heck of a lot of money, even a fixer upper as they can often need a hell of a lot of work being 49 years old.
If you're serious, you'll have to save a lot of money, like the equivilant of saving for a house but probably even more so, and that's if you can even find a Miller Meteor nearby, often people have had to get them shipped.
As for the siren, whilst the siren housing used in the film has been identified (although it itself is rare) it didn't make the sound heard in the movie, that was created by Columbia's audio effects department.
And PKE Meter nozzles for a mini bar? Adventurous but hardly practical, and practical's a thing to keep in mind with the car.
For a better idea of the work you'd be undertaking if you had bought a Miller Meteor, check out the sort of things Sean Bishop went through with the Superior he had for his first car:
Ecto RestorationNot to mention the extent things went to just to get his newer purchase (the Miller Meteor) up to scratch before he's even added any of the Ghostbusters mods to the car:
Flickr: Ecto 1Flickr: Jnghost1Flickr: Ecto1Flickr: The Ghostbusters car!!! Flickr: Ghostbuster Car Flickr: Bustermobile Flickr: Who you gonna call? Marina and Gina.It is not a project that can be taken lightly.