Asbo Johns / Aeromans previous series 2 is up for sale (tvr style flip paint) could be a good buy. Don't know its recent history though.
Price wise you are looking at roughly a 15k spread between a series 1 and a series 3. Ownership costs are low and depreciation on used is negligible. This gets dangerous though- each step to the newer car is just a few k and there is not much risk putting your money there. Be careful with your man maths or you'll end up in a supersport or Aeromax!
Having owned series 1 and 2 I would say in summary that the series1 is the purer drivers car, but is more hardcore for touring. (Less so than a plus 8 obviously). The series 2 is a better car for long distances and the cabin is more resolved. The extra power is noticeable but the purity of form is reduced. Looks wise the series 1 with the original roof assembly that folds flat looks the best I think.