Hello Rui- As you know, that's a fair amoutn of work there.
For market value when you are done- something like this as a ballpark-
Take the single family price per SF that is the average in Andover. Then take 75% of that number. Use that to multiply your extra SF-gla that you are adding/ plan to add. I think that'll give you a rough ballpark for "market value".
On that note, I assume you haven't bought it yet? If that's the case, there are a couple of good mortgage options out there where you can borrow most of the money needed to do this kind of home improvement as part of your mortgage amount. If you'd like more details on that, please let me know. Thanks and good luck,