What is the purpose of “de” in...

Fixed expression 'avoir l'intention de ' - fixed expressions are often best learnt as such rather than necessarily attempting word for word translation.

In this case:

"J'ai l'intention de le gater et .... de lui acheter .... "   - "I intend/mean to spoil him and ... buy him (des bonbons)...". The 'de' needs to be repeated before each infinitive that forms part of the statement 'avoir l'intention de'.

I don't think 'en' would fit here even without the need for 'de'- at this stage of the story, the 'them' she plans to buy 'some of' is not known

It comes from: j'ai l'intention de le gâter et de lui acheter des bonbons.

So, it belongs to the phrase avoir l'intention de...

Chris and I strike again at the same moment!

Ahhh - great explanations - thanks to both of you :)