I'd reiterate agreement with Forstbetrieb, that unless there is something not revealed in this itinerary a car will be a major headache. If arriving late and needing to stay near the airport nothing comes close to the convenience of the MXP Sheraton. It's attached to the airport building, and you can be in bed 15 minutes after exiting passport control. All other hotels require you to get euros or go to the tobacco store and buy a phone card, call the hotel, explain where you are, and then wait 20-30 minutes for their shuttle. If you don't know how to use pay phones in Italy, you'll have to stop passersby for help.

From MXP to Lago di Como is easy. The Malpensa Express to Milano Centrale is right there at the Sheraton. Como, one of the cities that ring Lake Como is somewhat unsightly, touristy, nothing special, and in my opinion, is best avoided. Instead, consider that from Milano Centrale you can take the regional train to Varenna. It leaves frequently, and only costs about 6-7E. I agree with JClasstraveller, the best way to get around Lago di Como is by ferry, not by car.

Getting off the train at Varenna you will be 1-2 blocks from the water and a ferry station. You can take a ferry to anywhere you want to go. For example, Bellagio is 5 minutes away by ferry. You can take the ferry across the lake to Cernobbio, Menaggio, etc. Great, fantastic ferry rides, with scenery galore. In my opinion the two cities to avoid on Lake Como are the two at the tips of the inverted Y of the lake; Como and Lecco. No reason to go to either one except for passing curiosity.

Having a car could turn a trip to Lake Como into a real pain. After visiting Lake Como you can go back to Milano and rent a car for the rest of your itinerary if you think it is still a good idea (it's probably not).