Unless it's dirt cheap, I'm not sure it's worth the risk. The Ford Focus is a very popular car and there are hundreds for sale on the used market, which means you do not need to take a chance on a car with a history of major problems.

Turbo failure can be caused by a range of things (oil seal problems, piston ring failures etc). I'd be concerned that the underlying cause(s) have not been resolved. This generation of EcoBoost was also subject to a recall for its coolant pipe - the owner should have paperwork to show this has been replaced. If they haven't, you could face some large repair bills if the pipe fails.

For all reported problems with this generation of Focus, see: www.honestjohn.co.uk/carbycar/ford/focus-2011/good