Le Marche is a quite mountainous region, and I would only look for fresh seafood on the coast itself. While the coastline of Le Marche is not long, it is filled with town after town after town -- so where are you thinking of going?
The rest of your question is so broad, it takes a book to answer. Fortunately, there are such books. Fred Plotkin's "Italy for the Gourmet Traveler" is a good investment, and you can look online for other books and information about the specialties of Le Marche. The Slow Food guide to osterie in Italy is also a good travel investment. It is written in Italian, but it is easy to decipher the recommended dishes at the recommended restaurants, and it focuses on local specialities.
We had a fantastic dinner in Gabicce Monte at a place called Posillipo in September 2007.
We were part of a larger group (14 people maybe) and I had no idea that the restaurant was attached to a hotel. The menu was chosen for us - and every morsel was delicious. And the view was gorgeous.