> I might potentially be hired to do some contracting (both development and administration) on a software system. Knowing my client very well, I am a bit worried that I might end up without any clear goals for when my job is finished.

If the requirements are vague then don't agree to a fixed price and charge by the day or hour instead. Ask for a small task that needs to be done, provide an estimate, perform the work while keeping the client up to date if the estimate is still accurate so they can decide what to do next and go from there for each task. Hopefully trust will build up both ways as you work through tasks.

You could both burn a lot of time writing up contracts for every small task instead of just getting on with it.