I was also looking at a second hand 12c but instead have ordered a brand new 540C.
Its the same basically as the 570s but with slightly less power (still way more than you need) and standard brakes rather than carbon ceramic (which I see as a benefit as I wont be taking the car to track days and carbon ceramic cost a small fortune to replace)
While I think the 12c is a lovely shape that has aged very well, the new supersport series are supposed to be the best 'drivers cars' yet of the mclaren. It was an easy decision.