I'm Kevin from Exomotive - thanks for posting and asking questions about our line of kits. We are still very new in the US, but our Exocet has done very well in our first year due to many of the reasons that Tony points out in his post. By retaining all the running gear any builder can tap into the 20+ years experience of the Miata aftermarket and bring the car up to the level he desires. Even with an old 100hp 1.6L out of a 90-93, the car is a heck of a lot of fun due to the light weight. Throw a V8 in there and you are in hypercar territory from a power-to-weight perspective.
We've also completely redesigned the Exocet from the subframes up this year and will be launching the new model in the next month or so. Our engineer has teased out some of the new details on the GRM forum here: GRM Exclusive Reveal: Exocet Spec-A: Grassroots Motorsports forum: Grassroots Motorsports Magazine
The most inexpensive Exocet I've built personally came in at just under $9k including a '95 donor car. To keep the price that low, the assumption is that you'll use the Miata seats, belts, wheels/tires, etc. The majority of builders this year have been in the $11-14k range all in after installing some nice wheels/tires, race seats, harness, steering wheel, engine/clutch maintenance, etc.
Re: registration - all our kits were originally designed to pass the UK's stringent Individual Vehicle Approval test. Each US state's registration requirements vary slightly, but are largely focusing on safety like seat belts, lighting at the correct heights, mirrors, and of course, taxes. We issue a MSO with each kit that you will submit to your state to register the vehicle and have a new VIN/title issued. The SEMA Action Network has done a great job of assembling a resource for each state and you can download a PDF with all the laws and forms you need on their website here: SEMA Action Network (SAN) TAG_TITLE_TOOLBOX
Driving an Exocet is just like driving a Miata, but one that weighs about 1450 pounds. The exposed nature of the experience is quite different for those who have never been in an exocar before. Closer to a four wheel motorcycle than a car.
The Mevabusa is quite a kit too at such a low weight and we should have our first of those in the US soon. If any of you are in the Atlanta area, give me a call and come by to see the shop. Let me know if you guys have any questions.