Megasquirt II or III. I think III would be the best.
In terms of price, I don't think you can find anything cheaper. In terms tuneability, MAP tables, fuel tables, timing tables are all tuneable by you. The tech support is fast and bear in mind, that the tech support guys are the guys that designed and built the software and hardware.
Check out the site. FYI, they will answer all questions you might have prior to you purchasing!
I recommend the preassembled units - quicker.