In regards to nuclear weapons, there is as yet no evidence that Iran are actually building nuclear weapons. As much as the world may claim that Iran are intent on this, there is as yet not a single element of evidence to support this claim.
However, if Iran weren't intent on building nuclear weapons, their current behaviour makes very little sense. By allowing unrestricted IAEA inspections, Iran could quickly extinguish these rumours, and make headway towards lifting the sanctions that are crippling their economy. They could even continue a nuclear energy/medicine program. I can't think of a rational reason why they wouldn't do this, except do want to make nuclear weapons.
If Iran had nuclear weapons, would they use them offensively? I don't know. I'd like to think no rational agent would ever use nuclear weapons against a country that could respond in kind. However Iran's behaviour is a little short of rational.
Is invasion the answer? Probably not. The nuclear program has broad support from both sides of the Iranian political spectrum, and threat of invasion may even increase the desire for a deterrent. Israel doesn't have the resources to occupy Iran, and the US/UK have no taste for another long war. At the same time, a tactical strike to remove Iran's existing facilities will do nothing but delay their progress by a few years.
Conclusion: It seems logical Iran are considering nuclear weapons, but an invasion is not likely to help in the long term. Solution: buggered if I know.