Mortgage or Business loan to couple 58 and 48 years old?

Planning to go into early semi-retirement with my wife in about 12 years time, and buy a house with holiday flats to let - so effectively with a salary. Could a mortgage be made available based on the revenue of the holiday letting business? What duration mortgage could be obtainable, i.e. could my wife's younger age allow a 15 year mortgage to be taken out? How do we work how much could be borrowed, do we have to assume equal salaries or could my wife effectively be the sole earner?