I do not reccomend any swaps. It is easier said than done and is certainly not as simple as taking out an engine and putting in a new one.
I am surprised at the Poster already thinking about switching to a Diesel engine and he has not even bought the vehicle
Get a handle on urself dude. Save urself a lot of trouble. and dont ruin ur future PETROL Surf just because u have the money but not the brain cells.
Simply buy a diesel Surf and remember the statement that i made earlier "it is easier said than done?" Also remember that the vehicle is not likely to feel like ur friend's original diesel surf no matter what u do