I would start with Ghidra , as its featuring a built-in ASM. For making a start, its perfect.
If you are more advanced into this topic, here is some starting point with a full toolchain designed
for MED9. The principle will work on any ECU, but as you are talking about ME9, you wont need to adjust much, as the C-Compiler should fit.
https://github.com/EliasKotlyar/MED9Toolchain