I agree with the high-side estimate.
Also impossible to know how reliable any given Caterham will be. I would favor those that have been used regularly over cars that have sat for years ("Low mileage!"). Seeing and driving the car will tell you a lot. And if you can test a couple, then you have a point of reference for what to look for, how they shd drive, etc.
Typically, a Caterham is pretty reliable, once fully sorted. I have zero mechanical skill and was always able to trouble shoot/fix whatever came up on a trip.
Pluses in the X-flow models are 5 sp, Dedeion, long cockpit, FIA roll-over bar, non-bench seats, full paint. The Craigslist car seemed to have some nice engine work wh/is always a plus as well.