The Austin-Healey 100 is, of course, the original Austin-Healey. It was built 1953-56, and although a straight six-cylinder engine is usually associated with Big Healeys, this model is the exception with only four cylinders.
The 100 came in two variants: the early BN1 Series cars (10,030 built) with a three-speed transmission, and the later BN2 Series cars (4,604 built) with a four-speed transmission, both with overdrive. After that, there are not any significant differences within this model.
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Transmission
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Whether the transmission is three- or four-speed is not a significant consideration for the occasional driver or show car. However, the three-speed transmission of the BN1 is somewhat weak and spare parts are rare. For these reasons, many owners have either retrofitted a four-speed transmission from a BN2, or they have modified the original three-speed transmission by removing a shift linkage-blocking device, installed by the factory, and thereby obtained four forward speeds. Avoid a BN1 with a retrofitted four-speed transmission if you are looking for a car that is strictly original, and avoid one with an original three-speed transmission that has been modified to provide four speeds since these transmissions will often be the weakest and most in need of attention.
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The 100S and 100M
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We should also mention here two special models: the 100S and 100M. The 100S is a racing model, and it is exceptionally rare (just 50 made) and expensive. It is not typically considered a beginner's first choice, and unless your budget is into six figures and you have access to very knowledgeable and experienced advice on the model, do not even consider one for your first Austin-Healey.
The 100M is a variant of the standard 100 with increased performance. It is a very desirable model, but due to much confusion and misinformation over what constitutes an authentic 100M, it is best that novices either not consider it or get the advice of someone who has experience with and knowledge of this model. There are many pitfalls here, and many cars out there are promoted as being 100M models when in fact they are not.
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