Well I have been a vectra tdi owner for 5 years (51 plate). What you describe sounds more like a Air Mass Sensor fault. ECU is a digital circuit and in most cases when it goes, it dies completely. So if your car starts and runs normaly than look somewhere else. Air Mass Sensor is that black plastic tube with a plug and wires coming out of it between your air filter and turbo. It's common knowledge they need replacing every circa 60k miles on tdi from that period.
Genuine one costs 60 pounds, refurbished about 20 and you can replace it yourself in 4 minutes with a screwdriver.
On the motorway when you reach a certain load (faster with your foot down, slower when gentle with accelertor) ECU gets a wrong measurement from the sensor and the fault cus the engine down in to a sefe mode. After turning the ignition off and on the engine light resets. Sounds familiar?
Generally:
ecu - won't start, weird behaviour, no power all the time
air intake problems (ams, turbo, crack in the hose) - no power, safe mode in the same situation
fuel problems (pump, injectors) - jerky and uneven engine running,
engine block (librication, cooling) - smoke screen behind
These engines are considered unbreakable and 300k are not uncommon.
Try another mechanic before you start spending this amount of money, fuel pump replacement is 500-600 pounds and a used engine 800-900 quid so there is no point.
