Assuming the inception date, company rep and lead attorney are in custom fields or else where in timeslips they can be added to the invoice layout. If you have never customized a layout this is something you might benefit from spending a little time with a consultant. Basically you open the layout template, locate the fields in the right section and drag and drop. The fields can be added to the header or a customizable section.
I don't believe there is a way to get YTD billing on an invoice. You can get since inception in a customizable section but can't filter that for date.

Caren,

What do you mean by this? "You can get since inception in a customizable section but can't filter that for date."

In a customizable section of the bill layout you can get the amount billed since inception. However, you can not get the amount during a particular period as there is no filter.

If I create a project and start billing to it then the amount billed since the date of inception would be the same as a year to date total. How would I add the amount billed since inception?

Its a field you can add to a customizable section of the bill layout. It's under client history.