Aloha Wendy,
Darin and I aren't there yet, but we are doing a ton of research on just this thing. I will tell you what I have heard.
I hear that right now it can be cheaper to build than buy. You would want to have everything well thought out prior to signing the deal because if you change things mid-stream, that is where they can gouge you. I will add a link to David and Katies blog. They just moved into their house they had built in Eden Roc. They documented the whole thing and there is a lot of lessons learned.
Moon Over Hawaii
If you are geared towards general contracting it yourself, you can save $20-30,000. It is said to put a lot of stress on a relationship. Bob Villa says that the two biggest reasons for divorce are finances and remodeling.
There are many home package builders on the BI. Here is a site that lists most of them.
Big Island Packaged Home Suppliers
and this one gives you owner/builder info.
Big Island Owner Builder Information
a lot of the package home sites give info about doing it yourself as well or getting a contractor.
Let me know if you have any other questions.
Aloha,
Sharlee