This is a very broad question.
In general, you should plan out what the things you need in the ECU and evaluate if a STM32 chip can do these. A good way to start would be to list all the interfaces to external devices that you need to support.
You've listed two of these. I see the DTI needs an RS232 input. The STM32 can't do this directly, but you can use a chip to convert between the single ended UART signal and the RS232 signal.
If you have no experience with the STM32, a good starting point would be to pick up a Nucleo board, flash some LEDs on it, and then move on to implementing the higher-level functionality that you need.
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