Most if the machinery could be tried on the moon first. If we adopt a Mars Direct approach, a return vessel with in-situ refuelling capability would arrive and recommissioning itself before the crew even left Earth, and we might even trial the return vehicle in an automated there-and-back mission first. With all these factors, I think the chances of success could probably be improved to perhaps 90% arrival and 70% return. Pure guesses though, of course.