My understanding is that monthly premiums should be on the order of $600/mo for an excellent plan with a low deductable and copay. It's more if you have a family, of course.

A standard part of the financial model is an emergency fund that exists for purposes like this. I believe the advice is at least 6 months worth of expenses in the emergency fund, which should be enough to cover deductibles in the vast majority of cases.

Generally, people who FIRE sensibly build quite a bit of slack into their financial models to make allowances for unexpected large costs.

Well, that's what the HSA is intended for =)