Most of the marine ply I see here in Michigan is made of Canadian woods. You might want to recheck that. Typically plywood is made of the cheapest available wood, and endangered rainforest wood is way out of that category, those woods are usually used for finish veneers. What is marketed now as Luan mostly comes from either Brazilian or African woods, not even close to Luan. There is a 'Luan' 3/8s " ply that is marketed as Luan for placing above the underlayment of flooring, its actually a Brazilian wood, which lets it retail ~ $10 a 4' by 8' sheet instead of the $ 90-120 + it would be if actually Luan. Luan and Teak are both now rare, and are now more generic names then actual woods. That said, there are actual rainforest woods, that are harvested sensibly in Brazil that are as good or better then both Luan or Teak. But, I would make an educated guess that the majority of the 'marine' plywood made in North America is fir. And its good stuff.
Kits will be much more expensive then buying the material yourself and building to a plan. The size you are talking about you could build in a long weekend once you have the material assembled. The wood you could pick up at Home Depot for ~ $200, plus fasteners, glue etc. And you'd have a solider boat. Sails, rigging, mast will bring her up to the $ 500 but you'd have a new boat.