I raced this Caterham for 3 seasons. K-Series, 150bhp (which was the limit for Mega class in Caterham Graduate series).
I then sold it through the tech team that supported the race teams. They sold it to a chap (after re-fitting lights etc) who wanted a fully sorted car for track days, and with the race extras, eg. proper seat, fia roll cage, full harness, straight cut box, plumbed fire extinguisher etc etc... many of the Mega class are built on a road car, with non-essentials removed - so it's easy to put back on the road.
Have a look at what McMillan Motor Sport have for sale, and the Graduates Series web site... ex-race cars are cheaper than road cars, as they will have a few scratches, but buy the right one and you will have a very reliable track day car... mine has been giving very good service to the current owner.