You will have 30 days from the time you move to PR to change your license. In order exchange your license from any state, the license needs to be current. You can register a car in PR without a drivers license.
I will recommend go to the CESCO office with the owner of the car to do a change of ownership BEFORE you give any money. Any tickets given to the vehicle will become your responsibility if you purchase a car and it have fines. Going to the CESCO office will prevent you getting stock with a vehicle that have fines. Look in the forum for other folks that had this happened to them.
If you buy the car from an authorized dealer, they will register the car for you. But spect to pay more at the dealer.
You can find good deals in the "clasificadosonline", just do you homework...good luck!!