hello I am looking at buying a petrol automatic 2.5 or 3 litre can you please tell me any problems or faults I should look out for, also can you upgrade the media system to a modern head unit thanks for any advice 

Welcome aboard! Biggest thing to check is the sills, especially at the rear. They're hidden behind plastic skirts and can rot spectacularly.

Other than that, all the usual checks apply when buying a used car. There are some age-specific things, the early cars use different ATF for a start and using the later ATF will result in poor shifting and fault messages on the dashboard.

Other things changed on VIN changes too and some parts for early cars are now NLA so work out if your prospective purchase needs anything and if it's NLA you may have to get replacements made as a one-off

Other than that, they're great cars, mine always puts a smile on my face!