What LEGO sets/blocks to buy?
Different kids are motivated by different kinds of sets. Some kids need directed building to focus them, and so a set with instructions helps them. Other kids just need useful parts.

I'd recommend getting basic bricks to start, something from the Classic line. Or else get something from the Creator line, many of those sets come with instructions for alternate models. This is a good lesson on how the same inventory of pieces can make more than one set. On Lego's Website you can check the set descriptions for Creator sets that are "3 in 1". 

Larger sets will cost more, but the models will be more challenging and will offer more opportunity to build more things (as it's usually easier to build things if you have more pieces).