Every big money (I don't know what you're finances are like, but I'd consider these big money) transfer has an element of risk. These all seem like players who could justify the money spent with the right development. The Brazilian one looks pretty immense.
it's jumping reach now, not just jumping. so height has nothing to do with it anymore, all in the jumping reach.
If you're a PL team I guess those are decent buys, brazilian especially but he cant join until 18 as you say. But I wouldn't say the 2 others are "worth it" generally speaking...
does the 15 yr old AMC fit into your style of play? what's his report card look like? his professionalism/other positive or neg personalities.
he's a very small and weak player but seems to could be really good technically,passing etc.
the brazilian also looks great but waiting 2 yrs is really boring..