I'd think that you should aim for one with the 1.4 16V petrol engine as it is the more reliable, I'm not sure what changes there were at facelift as my wife had a 2002 9N Polo, so same as pre-facelift in most ways, my older daughter has a late 2009
Ibiza so one of the first of the MK5 versions with the 1.4 16V petrol engine and it is still running okay at 56K miles - owned by her from new. It has had its front and rear springs replaced due to snapping, and is about to get its steering rods and so TREs replaced by me next month as there is noticeable "slack" in one side causing annoying and supposedly alarming feeling at higher speeds!
I maybe should not be saying this in this section of the
Ibiza forum, but if there is anyway you can change your plan and get a MK5 version, you and your son will not regret it, I have always felt that this MK4 version of
Ibiza and its Polo equivalent, just had too much in common with the first version of Skoda Fabia, I suffered that for 13 years with my wife's 2002 Polo 9N and hated it for that, it turned out to be a very nasty cheaply built car with all the 3rd world materials and running gear from the Skoda Fabia - and I and probably other found this out too late, so just lived with it and spend a lot of time and money keeping it safe and on the road. Inevitably some elements of Skoda Fabia were carried on to the next versions and even some on to the previous versions of these Ibizas and Polos, but I think that VW Group had by that time started to see that doing a cheap bit of
badge engineering had caught up with them so slowly moved back from that and probably the newer Skoda Fabias benefited from that as well.
If you accept my advice to try to grab a later version of
Ibiza, then again aim for the 1.4 16V engine, yes it is boring, no you can not remap it and turn it into a monster, but it will keep running without needing to rebuild it.