Some general advice: pay careful attention to your timing requirements. You will need to make sure that the microcontroller you choose has good enough performance to keep up with the requirements for changing the outputs, handing the LCD, reading and processing the ECU data, etc. plus a safety margin.

Not all 8051s are the same. Some have a different ratio of clocks to cycles, some can operate at high speeds, etc.

Even if you are not planning on using certain things like an LCD display now, I would recommend you pick a microcontroller that can handle it, so it will be easier to add in the future.

Andy