Hi Josh,
Back to your original questions, there are in fact several members here on Locost USA designing and building cars with similar dimensions to a Lotus/Caterham versus a Locost and its variants (i.e. McSorley, Haynes Roadster). One of the main differences other than overall size (a "book" Locost is about 3" wider than a Caterham S3 and about 1" narrower than a Caterham SV: a "book" Locost is about 4.75" longer in wheelbase than a Caterham S3 and about 1.5" longer than a Caterham SV) is styling. Lotuses (Loti?) and Caterhams have a longer bonnet which pushes the scuttle further rearward. Also, the tubes along the cockpit are angled downward to the rear versus horizontal. Finally, the upper and lower side tubes at the rear are curved and not straight. I personally prefer the lines of a Caterham over those of a Locost, but hey that's just me.
In my case I am using the Caterham SV size chassis for inspiration over the S3, not because I need the extra space in the cockpit (I am 5'-7" and only need to lose 5-10 pounds according to my "honest" mirror
), but rather to fit my not so svelte Miata 1.8, its transmission and perhaps one day a Garret GT2560R turbo I have my eye on. This insight is based primarily on the experience of Skip Cannon who shoehorned a Miata 1.8 turbo into a Caterham SV as documented here
http://www.britishv8.org/Other/SkipCannon.htm.
I also purchased a nose cone from Caterham along with a couple of rear fenders (with no questions asked on the part of Caterham other than what my credit card number was). They were about twice the price of equivalent Locost bits available here in the USA.
As mentioned by OzGecko above, making an exact copy of a Caterham chassis will lead to some expensive consequences. The Caterham uses a modified version of Triumph uprights at the front. On IRS cars, they use a Sierra differential and custom machined aluminum uprights. All of this can be sourced from the UK for a price of course. If you went with a DeDion rear, you could modify a live axle to work around the Caterham's pick-up points.
If you are interested, my build log (more of a design log at this point) is here
http://www.locostusa.com/forums/viewtopic.php?f=35&t=8885. If you could in fact pull some dimensions off of the Caterhams in your town, I would be very interested.
Finally, I hope I don't get lambasted by an Admin for using a 300 pound car engine and not being fanatical about what my, repeat my, car ends up weighing....