Well, looks like Japan might change its constitution thanks to North Korea's missile program.
In the 1980s, Japan threatened to start developing nuclear weapons in the event that North Korea did. Neither China nor the Soviet Union protested, even though both were opposed to nuclear proliferation. I'm guessing that the Japanese would have bought Soviet nukes or hired Soviet nuclear scientists.
The Soviet Union and China put an end to any North Korean nuke program.
I'm not sure about back then, but these day's isn't Japan the poster child for, 'we can have nuclear-tipped ICBMs within 6 months of deciding to have them, maybe sooner?"
Saudi Arabia is said to have the ability to get nukes ASAP due to close relationship with Pakistan, but it's not the same as having ability to quickly assemble it on its own.