welcome to WebmasterWorld, Metzed!
This is a subject that comes up once in a while, and there isn't one comprehensive answer. You know your business best, and only you know how many ingredients would spoil if you're not around to keep them going. But there are some common points:
1) keep your passwords somewhere safe other than in your noggin. Make sure at least two people know where that safe place is
2) have some friends who can act as a specialized executor: people who know how to transfer domains, close AdWords accounts, pay out your invoices, etc. Or make sure you have someone in mind that will be able to take over your whole business should you meet an untimely end. This may involve a clause in your will specifying transfer of ownership, with provisions so revenue/profit may continue to provide for your family & heirs as well as the new business owner.
This did actually happen to someone I know. Their life ended unexpectedly and the business they owned (a larger operation with many employees) was left in the care of a close colleague in the same industry, with clauses that provided dividends into a trust fund for his kids. Adult family members of the deceased acted as an ad hoc board of directors representing the children as all the details were worked out... it was complex and tragic but worked out well in the end
3) Document EVERYTHING about your business that you can. Everything!
I'm in a similar situation - I have a few complex online businesses that are run 100% by me. And I'll admit, I am not as well-prepared as I should be. I do some of the above, but not as thoroughly as I should.
Ideally you would like to have a backup plan, without paying a retainer. But that may not be possible if you need risk assurance for your clients.
Perhaps the simplest path is to grow your business up to the point where you require a partner/apprentice/employee, and train them so they are capable of continuing operations in your absence, or at least knowing what accounts need to be settled and what relationships would need to be maintained or terminated.
I'd love to hear how other sole-proprietor webmasters handle this