First off, I'm assuming you're talking about World Nomads Insurance?
You can't purchase their insurance more than 365 days out, so buying now is not an option. You'll need to wait until at least May.
Also, the policy limit for the more expensive policy for Trip Cancellation is $10,000 (the less expensive policy is $2500). I can't find if that's per person or per policy, but it's usually per person. But for Japan, I can't imagine your trip costing less than $10,000 per person. So you might not be completely covered if you had to cancel your trip for a covered reason.
I was also wondering if there was no pre-existing condition clause or no pre-existing condition waiver?
I looked at World Nomads' website and found this explanation (bolding mine):
Policies purchased after Wednesday, 7 June, 2017 5:00 pm CDT.
Pre-existing conditions – unless they are controlled and stable - are not covered for Trip Cancellation, Trip Interruption, Accidental Death and Dismemberment, Emergency Sickness Medical Expense, and Emergency Accident Medical Expenses.
And not to get too technical but a pre-existing condition is defined as a something that you have showed symptoms of, been treated or tested for, recommended you be tested for, or received a prescription for in the 90 days prior to the effective date.
If you have taken or received a prescription in the previous 90 days, the pre-existing condition doesn’t apply if the problem is treated or controlled solely by the taking of prescription drugs, and if there has been no adjustment to that prescription in those 90 days prior to your policy date
So it looks like there is no coverage for pre-existing conditions unless it's something like high blood pressure or diabetes that has been medically stable via medication, with no change in prescription for the 90 days prior to your policy effective date. So, in general, no pre-existing condition *coverage* and no pre-existing condition *waiver*.
If you don't care about pre-existing conditions or a $10,000 cancellation limit, then they seem to be a good company, with a good policy. And the price seems to be really good. And you may as well wait until later to purchase it, because pre-existing conditions aren't covered no matter when you purchase it.
Sayhello