It all depends on what you want to do. If all you want is the typical powertrain codes for the engine and transmission, a basic OBD-II scanner will work very well. My personal favorite is the MS309, which you can get on Amazon for about $20. Small, basic unit, fits under the seat so it's handy if you care about that. No other pieces or equipment necessary, it will read and clear codes.

If you like tech, and are an android user, you can get an ELM BT interface for the OBD-II and download the TORQ app from the Play store. Or you can get a similar setup for a windows laptop. This will also give you some live data, for a bit more than the basic handheld unit. The con of this is that it's a bit more cumbersome to use, so you may find yourself procrastinating.

And as you dramatically increase your expenditures, you'll get more features and interfaces. Heck, if you want to manage all car systems you can buy SDS (or you could just buy another ML, ha ha).

That said, if you aren't in California you can just go by Autozone or some other parts stores and they will read codes for you. If so, be sure you write down the actual alphanumeric codes, not just their summary or diagnosis. You can search them or post them and we're happy to give you a hand.

Good luck.