Talk to an attorney who deals with contractors. Our state requires licensure, so there is a lot of regulation surrounding it that a lawyer should be familiar with. Your state may have specific requirements for warranties.
A lot of what you ask about (terms & invoicing) is governed by the UCC...
There are some decent deals on Axis and Pelco PTZs and Arecont 360s that show up on eBay. What brands would you source used cameras from? What are potential pitfalls?