Buy rental car or demonstration car?

How do you think about this? I find well-equipped “yearly cars” at many Fiat dealers, mostly around 20tsd km, shortly before TÜV-falliness (that is after one year) or with fresh TÜV for another two years. So they are vehicles from car rental companies. On the other hand, TÜV have demonstration cars for 3 years, are often younger than 1 year and sometimes have a little less mileage. I had now carefully driven my Fiat and it runs accordingly well and sparingly. I often say that they “borrow” a demonstration car and then “step” it over the weekend. They don’t care if the car has 20 or 2000 km down. Car rental won’t work any other way. Is the purchase of such a vehicle a risk, better keep your hands off it ???? Or do the new vehicles completely stop ???