If your flight is from MXP at 11:45 am, there is no need to sleep at the Sheraton. I would suggest you to arrive at around 10am in Malpensa - if you have status with any airline that gives you priority security, a little later is fine. Economy lines might be a little long sometimes but 1h45m should be more than enough even if you have to check baggage.
There's plenty of buses from Centrale Station and also trains that go to Malpensa. However, never take the last possible train or bus; sometimes traffic or delays can impact the schedule.