Specifically, what do you want to do? Do you want to interface with the powertrain controller and just read codes? Read engine data and diagnostic codes? Perform on demand controller tests? Measure electrical and mechanical systems? Perform waveform analysis or some combination of the these?
If you want something that can do it all be prepared to to pay nearly the cost of the mini. If you only want to read codes--$50 to 150. There are thousand of products. If you can be more specific I might be able to help.
You should also be aware that with just a diagnostic code and description you can't effectivly diagnose and repair a vehicle. The code only tells you what system or circuit and sometimes the type of fault. It doesn't tell you whats broken or how to fix it.
For instance, You may have P0118. An engine temperature sensor fault, open circuit. Is it the sensor, the engine computer, the a broken wire, hidden wiring corrosion or one of the 4 connector terminals corroded, broken, or just backed out of the connector or maybe an overheated engine? You'll need a DVOM, the correct connector terminal interfaces, a wiring schematic and a component location diagram. If there are 3 or 4 DTC's what then??
Very rarely is it possible to see whats wrong without experience and more tools. I've been doing this very thing professionally for nearly 20 years and often get cars with very simple problems that have been to 3 or 4 very reputable shops before me. Electrical parts are usually non returnable. A guess can be very expensive.