For ODB1, there is a circuit that need to be build (or a cable that need to be purchased). For ODB2, as you said, with an ELM327 adapter, its very easy. I would say its probably the easier beside ALDL and MegaSquirt (Megasquirt CAN being the hardest :P). That adapter can be Bluetooth or USB. If you wouldn't have already bought your Arduino stuff, I would have told you to look at
http://arduinodev.com/hardware/, he got a good collection of Arduino kit for ODB2. He got a pretty clean ODB2 Arduino library that you can look at
here (and still use with whatever Arduino you bought). Also, he has
this ODB2 emulator which seems pretty damn cool, no ECM bench test needed for testing from what I've understood, the emulator will emulate the whole ECM, not only the sensor like the JimStim does when we use it for ALDL or MegaSquirt. I might have to buy one at some point :)