The 1960 to 1965 Comets were based on the same platform as the Ford Falcon. The 1960 to 1963 models are referred to as "round bodies" and share the same basic shape.
The Comet was not even technically a Mercury car until 1964. It was sold at Mercury dealers, but the car itself contained no badging to identify itself as a Mercury, Ford, Lincoln, or Edsel.
The thriftpower 90 horsepower engines in the original Comets did not lend themselves well to high performance, and these cars were seen as boring by some.
The 1964 and 1965 Comets featured a major redesign that made the cars more square. A performance version called the Cyclone was also first sold during these years, replacing the previous S-22 car. I wonder if Mercury named these cars after Coney Island roller coasters.
The 1971 to 1977 Comets were based on the Ford Maverick platform. The base engine was still a puny and weak 100 horsepower I6, but that could be optioned up to a 2 carb V8 making 210 horsepower, a big improvement for a rather small car.
The Comet GT was the muscle car of the group during these years. It was only sold as a 2 door coupe. Other versions of the Comet were sold as 4 door sedans and even station wagons.
These cars, especially the early ones, are considered classics by many collectors. You can find several collectors clubs that focus on these cars and the Ford Falcon and Maverick. If you are looking to buy one used, try looking on eBay, where nobody knows what their junk is worth. There are also plenty of parts on there.