Payment method depending on categories of items purchased?

[Bernard Grosperrin]

I would need a little guidance to know where I should do my "surgery" 

I have different types of items I sell. For One category (and all it's subcategories), I want to offer PayPal subscription as alternative payment type. I have PayPal subscription "almost" working like I want, I just don't want this choice visible all the time. So I would like to be able to check the categories of items in the cart, and if ANY item has a category <> than the one for which this is OK, do not display, otherwise display....

I think I can write the code, just don't know where....

Thanks in advance!