This is a good question and I am glad you brought it up.
Here are my two cents. If you are setting out to start a business I'd say for best business practices it is always good to start on the right foot. In fact, if you are acting as a business, it is pretty important to portray yourself as one. This includes setting up your business as a particular entity, either a sole proprietor, LLC, etc. Checking with your county and township for home based business regulations/procedures. Determining your insurance needs (property, E&O -- which stands for Errors and Omissions). Getting service agreements together, etc.
Now don't be alarmed, because all of the above isn't all that hard to accomplish; it just takes some time and dedication on your part. In fact, sole proprietorship is the most simplest business entities to form. It actually doesn't cost anything, all you need to do is complete some paperwork. There are some other costs though, for example you will incur costs for registering your DBA (Doing Business As) name. You can than later change your business to a single member LLC, if you wish (again there will be costs for forming an LLC - check with your county and state for more information).
In terms of insurance, it is a good idea to check our your options, determine costs and your needs. Because it sounds like you will be doing bookkeeping, you should definitely consider getting Bonding Insurance and E&O insurance. There are some VAs on here that specialize in Bookkeeping, so hopefully they will chime in with some information on insurance needs for doing virtual assistant bookkeeping.
So, to sum this all up, if you are starting a business, making sure all of the legalities of your business are in place before you open your doors is important. While it is not necessary to have insurance right away, it is always important to understand what you are liable and responsible for whether you have insurance or you do not.
Starting a business is exciting and can be scary. Don't let any of the above deter you. Congratulations on getting a lead for your business already -- that is great news. Keep forging ahead; and you will do just fine