Get the cambelt, tensioners and waterpump done, and the Q2. It actually doesn't make a massive difference to do the clutch at the same time though, but driving around with the standard diff in is a timebomb waiting to happen. Cost of new gearbox due to diff failure - £2500. Cost of Q2 - £500. Do you want to take the chance? My car has already had a new gearbox once due to diff failure, Nicky's 2.5 just shed its diff, he was only doing 20mph in a straight line at the time. Get it changed, is my advice