We have used Nadi-core for several projects and it is a good material. There is a lot of technical information at: http://www.nida-core.com/english/index.htm I would certainly consider a well built cored hull with Nadi-Core. But, I would suggest buying the entire boat rather than a kit.
There are several near new or well used boats in that size range with power which run from $18,000 on up to about $30,000 and are well proven hulls for the type of running you are considering.
Personally I would want something a bit larger than 18 to 20 feet for the Gulf (see that there is an 21 foot Mono hull missing today in bad weather). There are also some nice 25 to 26 footers, which are not much more money, and a lot more seaworthy.
The AeroCat 26 trawler looks interesting--only $75K complete with twin 50 Hondas!
Bob Austin