The minimal hardware you'll need is a freeduino/arduino, LCD, and some electronic components. dcb has done a great job listing most of the hardware that will be needed. Take a look at the "realease one workplace" thread. There you will find all the information you'll need.
I'm planning to connect to my car via the ECU. As diesel_john pointed out you can connect directly to the VSS and Injector. I don't know which procedure would work better because I haven't tried either yet.
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