I think $18K is high for an SRT6 with 70 k, I picked up my coupe 2 years ago with 26k miles for $14k. If you look around you'll find a better deal. If you drive the NA then an SRT6, you'll see why the 6 has been sold.........
A friend of mine found an SRT6 roadster for less than that with 30k miles last year .........
The thing with the roadsters is that only 928 were produced worldwide, but I'm sure there's a better deal out there