A lot of maintenance can be done with allen wrenches, screwdrivers, and crescent wrenches. I think the suggestion to buy tools as you need them is a good one. Depending on what bikes you're working on, you may need more specialized tools that you wouldn't have thought of, and you might never end up needing tools that many mechanics would tell you are essential. If you're opening a shop your needs are going to be different than if you're overhauling your personal bike.

Generally speaking, cone wrenches, cable cutters, a fourth hand tool, crank puller, freewheel and cassette removers, and a chain breaker are a good place to start.