Lancelot
I suspect it is possible to build an R1 with a starter kit base keep it light and prob get the price in under £15 k, however as Mike Bees suggests a conversion would be easier not least for the SVA potential aggravation. As you suggest even with De Dion (which may provide better diff ratios) and an R1 it would be quick. I doubt you lose much on the value if it was a neat converesion (STM, Kit Car workshop) to put the cage for the engine in and the pick up points for the gear linkages, prop, the rest is not difficult.
I would agree re J Whiting, I could not really understand why they backed off the project, I suspect having to use older engines to get through SVA, or even a nod from Caterham indicating not to produce a car for £13k that could take the piss out of the more expensive CECs in the range!