Oh, this is for a working holiday visa! That changes things.
The only people with WHV’s who need ARC’s are Britons. Some people might tell you you need one, but actually you don’t, and logically you would need to cancel your visa to get one (because the WHV would be replaced by a resident visa), but that would cancel your permission to work.
More information here:
As for proof of insurance, you need whatever the TECO is willing to accept. The initial period of stay is 180 days, so in theory they should accept insurance that will only cover that period if you really intend not to extend it (and say so in the application letter).
Please note the “180x2=360 not 365” problem explained in the other thread.