I think the 2.5's start showing up around VIN #964
You may want to look at this posting:
Roadster production dates
Also, note, I dug up a lot of info (looking at Carfax and such) on North American bound Roadsters, but have very little knowledge of the European/Asia/Australia sequence. If you only talk of the last 4 digits of the VIN then there can be another (but non-North American) Roadster with a VIN that ends in the same last 4 digits.
The European VIN -> Model feature and production count list is a bit different from what went to North America, so you would likely want to try to track them separately.