There were a total of 5 different generations of the Triumph Spitfire during its 20 years of production. They included the Mk 1 or Spitfire 4, sold from 1962 to 1965, the Mk II, sold from 1965 to 1967, the Mk III, sold from 1967 to 1970, the Mk IV, sold from 1970 to 1974, and the Spitfire 1500, sold from 1974 through 1980.
All Spitfires were sold with inline 4 cylinder engines, but they did gradually get bigger from the first model to the later Spitfires. The original had a 1147cc I4, while the last Spitfire came with a 1493cc I4.
These Spitfires were the economy sports car in the Triumph lineup, and they were designed to be the "base sports car" in general, of sorts. All trim was as basic as possible, and even many of the components seemed to be cost-saving measures. Even the steering wheel was made of plastic. The rear single transverse leaf swing axle was not arranged in a way that most drivers would have preferred, and it caused some serious oversteer when cornering hard, hardly the performance that a small sports car driver would want.
Later model Spitfires had added performance and safety components, and therefore are a better buy for someone looking to buy one of these cars to drive, which I think is what most Spitfire buyers are interested in. These cars just aren't rare enough to be trailer queens.
If you are thinking about buying a Triumph Spitfire, you can check out Hemmings or even Craigslist, but you'll probably just have better luck just going on eBay. You'll also find some good deals on parts on there.