Based on what I have seen here in the past year:

A limited Roadster with those miles would go for $18 to $20K.

An SRT Roadster would be advertised for $23 to $25K and sell for something over $20K

I'd say $18K would be on the upper end, pricewise for an N/A coupe.

Is it the color you want?
Low miles - is that REALLY that important?
Do you REALLY believe they have run the engine that much over the years?

If your answers are: "yes, yes and yes" then I'd say buy it. I predict that some here will tell you that you overpaid, but it's up to you.

I'd have the coolant flushed, brakes bled, and if it's an auto, the transmission fluid and filter changed, if it were me. And tires? Old tires even with a lot of tread are nothing I'd drive on!