You need to look around the locost websites because there are quite a few companies that specify in building "book chassis". or in other words taking the welding/ cutting of the chassis out of the build proccess. Most will get you a decent, unpainted chassis for about $3g in a big shipping crate. You will also be able to order a fiberglass nosecone, suspension parts or any other number of things you will need to finnish the car. Most kits now are being built around miatas, wich I see as totally positive. The miata parts are well built, and are a great basic platform for the cars good handeling. I personally have been thinking of putting one together with miata suspension, and a honda s2000 f20c drivetrain.
Should be an easy fit with minimal fabricating do be done, and stock it will get me to a very reliable 240hp. I would be using the honda engine, tranny, and then either going to a miata diff and halfshafts, or out to the honda diff with a resized driveshaft. I figure that the stock s2000 engine should get me into territory just inbetween the caterham CSR200(200hp) and the CSR260(260hp) wich will do 3.7 sec and 3.1 sec 0-60 respectively with less torque making it safer and easier to drive then both.
...just some more food for thought