Good Morning Tom, It's good to have the shop manual like the ones we talk about from ebay and or a subscription to AllDataDIY. The manual can tell you what tests to run or what to look for, parameters and such.
If you're looking for one, make sure it comes with the CANdi module and the 33.004 North American software. They'll also come with multiple adapters, power supplies and such and connectors used for self-testing. Many include the TIS2000 software which allows you to at the least download a snapshot and view on a computer screen.
My laptop does not have a serial port so I bought an adapter that everyone recommended and it works seamlessly.