Probably your best bet is to fly round-trip into Izmir and have your pre-arranged guide (with car and driver) meet you at the airport there and take you to Ephesus, which is only about an hour away.
We arranged for a private six-hour tour in the Ephesus area (but from Kusadasi port) using Ekol Travel, and our excellent guide lives in Izmir, spending much of the tourist season giving Ephesus tours. Because of the time of year you are going, you should be able to arrange this without worrying about tourist hordes. Guides are very reasonable in Turkey, and with a private guide you can decide how you want to spend your very short time, rather than waiting for the slowest person on your bus. Of course, in Istanbul itself, there is no need for any guides; just use Rick Steves' excellent Istanbul guidebook.