He cant base his life on the condition of spliting up
Let her move in give it a couple of months, assess it then if it is all going to plan get a car, a second hand one or jap import job done. And if the worst happenes between you and Sarah which IMHO is highly unlikely, then you will always be able to land on your feet by selling the car plus you getting a raise and your parents are always there to fall on worst case scenario