Trading for Prestia is a gamble. Every club pursuing him would have performed their "due diligence" and would be fully aware of his medical issues. The unknown is how his knee will bounce back and whether or not he will have any longevity in the game. I will not go into detail about his specific problem, but either Dion will make an excellent recovery and the knee will stand up to the rigours of AFL football for a number of years with no functional decline, or he will develop ongoing issues and never return to his best.
If the risk that Richmond are taking pays off, they will look like geniuses. If it doesn't, they will look stupid.
I would be staggered, however, if Richmond will need to give up pick 6 for Prestia. Gold Coast will know the risk Richmond are taking and the potential that he is "damaged goods". That clearly reduces his currency and I doubt Richmond would be foolish enough to waste pick 6 on a gamble as big as this one. Gold Coast will make noise and publicly make high demands to appease their supported base, but the reality is that I doubt they have much of a leg to stand on when push comes to shove at the negotiating table.