Triumph initially started importing bikes to the US for model year 1995. There were only two engines: the 3 cylinder 885cc used in the Speed Triple (and Tiger, Thunderbird, Trident, Sprint) and versions of the Daytona and Trophy which used that engine or a 4 cylinder 1180cc.
In 1997, the Speed Triple and Daytona were completely redesigned. The T509 Speed Triple maintained the old engine, while the Daytona got a new 955cc engine. In 1999, the Speed Triple got the 955cc engine (detuned from Daytona spec for more midrange). In 2002, the Speed Triple underwent a minor redesign, the tweaked chassis and engine result in slightly higher performance.
You asked for some thoughts, I'm no techie, or Triumph historian, just a fan of S3 since 1995. The 1st generation S3 are a timeless classic. Not the fastest or best handling, but in my opinion, the best looking. The only thing that comes to mind to watch out for is that some of the early 1995s had sprang clutch failures. Triumph solved the problem in the later ones with 6 speeds. If you're looking for "attitude" then start with the T5s. The early 1997s had factory polished frames and wheels, it was gorgeous, but Triumph decided that grey powdercoat was more cost effective. A few of these bikes also had frame failures. You don't have to worry about running across one of them, they were replaced under warranty. I can't think of any major systemic problems with the 1999-2001, although there were a few recall items. All of these bikes are well supported but getting kinda long of tooth. If that concerns you, then 2002-04 included a big drop in factory MSRP, and deals should be readily available next Spring when the 05s hit showroom floors.
The factory suffered a fire in March 2002. It was widely covered in the press:
article 1
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Happy Shopping!
