Could most of the tasks be done from the comfort of your employees home?
It has been documented that statistically employee's that do not have to commute to work are happier than those that do. They also save time and money for themselves and their employer working from home using their own resource's.
A Virtual Assistant works from the comfort of their own home. Invoicing you only, for the hours that they actually work.
As an employer you will gain from all of the benefits of having a skilled worker, and you won't be tied down with all of the tax and benefit issues which can be financially draining.