wallace, if it's genuinely a 2003 "03" plated car and it's 2.0, then it has to be a petrol....
as above DON'T DO IT unless you are utterly desperate for a few grand in trade in value that he will have to give you to part with your 2.5TDI.
There's nothing particularly wrong with the 2.0, but it's be my choice a long way below virtually any other engine in the A4 B6 lineup (I think the 1.6 1596cc petrol and 2.4petrol are the only ones I would want to own less)...
The 2.5TDI, despite being quatto and having twice the usable power has better fuel economy than the 2.0 petrol, even when you include the cost difference in petrol vs diesel!