I have been doing a lot of research into Miata based kits, and there are 3 options already out there for NA, each seeming to fill a different niche.
FM Westfield- A very complete kit for top dollar. The strength of the dollar has really hurt the pricing of this kit, as everything comes from the UK. I think FM Westfield's biggest strength is the amount of information they make available about the kit and the build on the Internet. You can read their build diary on line, seeing over 200 pictures of the build, you get a complete list of what you need from the donor and what needs to be sourced on your own, they will sell you a pallet with all of the donor parts needed if you want, and they have a forum were they are very responsive to questions from builders and potential customers. It is too bad that other companies don't go half as far as FM with their online presence.
Coveland- They have the best prices, but it is difficult to tell really what does and doesn't come from the kit once you get beyond the the big stuff. The frame is a copy of the old CMC frame used in Keith Tanner's book and it is unclear if Coveland has fully addressed poor front suspension design and diff mount issues that Keith dealt with on the CMC. Their Internet presence leaves a lot to be desired, as do the stories of people having a hard time getting a hold of them.
Deman Motorsport- Deman seems to be right between Coveland and FM Westfield in just about all categories. Their price is in the middle of the two, their kits appear more complete than Coveland but not quiet as complete as FM Westfield, and the online documentation seems to be somewhere between the two too. I am not sure how their communication is, but I have heard that they are pretty responsive to people building their kits. There biggest plus seems to be the ability to buy the kit slowly, in different modules. I like this idea, as on top of the fact that money goes out in pieces, I am not thrilled about the idea of tripping over fiberglass body panels months before I need them while doing other parts of the build.
If you think you can find a niche in there, I say go for it. In my mind, the best way to be successful would be to take pieces from all 3 companies, offering FM caliber support and documentation, Deman style modular builds, and a competitive price.