[Ken Lion]

Airbag light and handbrake are common faults with well-documented, straightforward and usually economic solutions on here.

There is expert help available for ECU problems (e.g. MarinaBrian) - and may also be economic, depending on the root cause (e.g. water damage in a drain-blocked plenum).

Is the car running and otherwise sound? Gearbox ok?

With a budget of £200 (without resorting to commercial garages) to sort out the named problems, £600 sounds a very good price for an '04 diesel if there are no other issues.