It looks like you are looking in Woodstock. There are many different areas/types of homes there. Is it in one of the newer subdivisions, closer to downtown or out near Bull Valley? I would have your agent pull up the recent solds. This will give you an idea of price range. Are there any homes with some of the same amenities you were thinking about putting in this home that have recently sold? If there are, that will give you a better idea of market values once construction is completed. There are some areas where you more than likely will price yourself out of the market. Also, have your agent give you an idea of the percentage of the construction you will get back....like a kitchen remodel will give you a certain percentage back of what you spent.
If you are seriously thinking about this home, you can bring a contractor with you before you write a contract so you will have a better idea of costs. Once you obtain this information you will be better able to make a decision on whether or not the construction costs will be worth it.