PR is calculated based on a car’s maximum demonstrated performance. It is a measure of “how fast a car can go” when fully upgraded and tuned by the best in the business. If you are aiming to eventually build a chart-topping car, you'll want to pick a higher-PR vehicle.
That said, PR is based on records set by other players in the past. This means that if a car is relatively new or less popular, its full potential might not have been discovered yet.
If there is no PR displayed in the shop, the car is a recent addition and hasn’t been maxed out by enough players. But even with cars that have a PR rating, it’s not uncommon for someone to find a better tune and push it even further!