All street cars leave the Manufacturing plant with a VIN. If they don't have one, they cannot be sold and used on an American roadway.
Given that it has 20K miles on it, it was obviously driven around a bit either by the dealer (demo car maybe) or another owner. Given the 30 years or so of the cars age, hard to really know what the true story is. All you can go by is what you have in front of you.
There is a VIN somewhere on the car. Fox Mustangs wore the VIN on each fender and body panel as well. If you open the door, pop the hood and hatch and poke around a bit, you should be able to find one of the vin stickers on the car.
You could buy the car, but without a clean VIN, you'd never be able to drive it on the road.
Also, be weary of missing VINs as it means it could possibly be a stolen car. I would buy a story of someone stealing the car at 20K miles and pulling the VIN's and never getting around to finding replacement VIN plates over a dealership donating a Mustang to a college. If it is a stolen car, you don't want to buy it.
Also, For pre-1987 Mustangs, T-tops aren't all that rare. They are a little more common than sunroofs those years. However given that most people like 87+ mustangs, they are rare when you lump those cars in.