On Modular: what is the safest and most efficient?

We are designing a modular home in the Northeast and have been struggling with price and preference on the materials to use. The choices are hard because the construction costs are very high in this part of the country. There are so many things to consider in this process and we keep coming up with new questions with each answer.

There appears to be no end to the options for factory-installed, very efficient insulation, but it's hard to tell where the best bang for the buck is. We are a little worried that a home that is too air-tight might not have adequate air exchange. A must for us it to have low-to-no VOC's and clean air. Is there a safe, cost-effective industry standard?

We want solar on the roof. Despite a significant initial investment, my math says that we can justify the bump in mortgage payments because the energy $ offset will easily be a net reduction in monthly out-of-pocket. Is this a fair assumption? Is there a reputable website that compares energy sources, their cost and benefits? I have heard that the government has something on this but don't know if this is the best place to look.

Lastly, we are stuck choosing between propane and oil. While propane is more expensive, we are leaning towards it because it is more efficient and cleaner. We will have the option of supplementing by using some small electrical units in common areas because of the solar if that works out. Thoughts on this?

Any feedback/ideas for this project?
Thanks.