Head Gasket. The white stuff is a air being blown in the oil causing foaming. There is a cheap fix, pull the head - replace the gasket and hope the head hasn't warped. More expensive is a rebuild, however, if that is what you pick you better really like the car because you will never recoup your cost.
Best bet - if you bot it from a dealer then they should have known about it and their failure to disclose makes them libel.
If priviate - well buyer bewere
