Mattb,
There are some interesting opportunities in the Nagua - Cabrera areas. Although Nagua is not a hot spot at present. I think these areas will inevitably develop. Especially since your investment time horizon is so long, you will definitely achieve super returns on your investment provided you get something decent and have professional people assisting you and helping to protect your interest in the land over that time.
Like most businesses the key to getting deals is having good local connections and dealing with reputable professionals. But your idea is right on. I am an attorney in New York and married to a Dominican. I invest in DR and happen to own a few parcels in that area and in Samana.
If you want to ask me anything more specific - IM me or email me a
kdelrington@gmail.com