In general, right now it costs more to build brand new on a lot than to just buy an existing home/ property. Unless you absolutely need a brand new home, you're better off financially to find an existing home where the seller is in a position to offer a low price.
Thanks, and Good luck,
Most wetlands in Massachusetts are protected under conservation laws. The Sudbury Conservation Commission would have a definitive answer for you (978-443-2209 x1371). I think Sudbury is pretty strict about their protected wetlands like my town, Norfolk, is. Trying to build on wetlands may be an uphill climb, if not impossible.