Personally i would say it depends how much joy you get from the S1

If you only drive it now and then and do alot of mile munching in the focus then sell up and buy the ML

You can always sell it ans buy another S1.

Or

sell the focus, get a loan for the ML and try it for 6 months, if you like it,selll the s1 and pay off the loan. may cost a few hundred more but atleast you will be sure, who knows, you may find away to keeps the s1