martiki6
It is definitely worth your time to call the assorted principal importers (Birkin & Caterham, and the domestics like Brunton). They may also know about second hand clams or other cars (shameless plug... Kitcat's caterham is a great!).
Another resource to check would be the locost folks. If you can't buy it, these guys usually know how to make it.
If you have not yet, I strongly recommend getting a ride in as many sevens as you can. You may find the car is not to your liking, or (like me) it'll be the final nail in your coffin.
Regardless of what you end up with (lotus, caterham, birkin, brunton, locost, westfield, etc....) do what makes you happy!
cheers.