QUite obviously this isn't for the average consumer in the least bit. Anyone who can afford to spend money on this likely already has a sports car of some sort which they love. And FYI, I don't believe the Ariel Atom or Catterham are street legal in the states, so aside from finding a track nearby to tow them to, you'd have to get ready to shell out on some traffic tickets. But, again, if you've got the cash for those cars you probably aren't wanting for money for a traffic citation.
Both are legal... and I've seen them both on the road in NY. You additionally could look at the Ultima GTR (my preference in the ultralight category), though it's more expensive. The way they get around crash testing and all that is that they're sold as kit cars (which may be why you thought they weren't legal). I don't think they can be sold complete in the states, you buy the parts and assemble them yourselves. For about $90k you can put together an Ultima that has 600hp and weighs well under 2000lbs, 0-100-0 as fast as a Veyron... and it's only got rwd instead of awd. These cars are pretty radical... which also brings up the RADICAL nameplate, another maker of ultralights but I'm not sure those are street legal (they make a V8 out of two Hyabusa motors, super light and makes over 300hp... very cool tech).