For standard Stage 1 calibration: Not needed.
For injector diagnostics: Genuinely useful — injector current waveforms reveal things no scanner can.
For BDM/BSL work: Helps verify signal quality.
What to get: 4-channel, 100 MHz. PicoScope 4000 series is purpose-built for automotive.
Bottom line: Start without it. Add it when you encounter a problem it would have solved.
Safety ECU Read/Write covers which diagnostic tools matter at each stage — so you invest in the right order.