There is so much to see in East Berlin, personally, I would save Pottsdam for a return trip, which I'm sure you will make with more time to spare.

If the private guide can get you to more places, it might be a good idea to go with him. Berlin Walks (with the group tour) is great for the overview of the historical and main sites and is a great deal. Perhaps you can do both?

It is hard...with just 1.75 days, you will get a flavor for this city, but will not be able to see it all. I would stick to things that define Berlin versus other cities, so you get a sense of the history of the city.