If you haven't done it before, make sure you read a lot of books and search the internet for horror stories so you know what to beware of .
Then get an attorney who has handled new construction before.
Build in to the contract a completion date and a fee for delaying your occupancy.
As for the $/ft, ask the town building department for help, call a few local contractors for their ball park, and try to see if a few local appraisers will answer this question.
Most real estate professionals are not as current on new construction pricing, because there is not much of it right now.
(Please note: when you choose an answer as a Best Answer, or at least give a thumbs up, it helps those who answer questions here.)