Signing Donaldson makes sense if you believe he can return to his 2017 Toronto form and probably makes sense if he returns to his 2014 Oakland form. Riley isn't ready. Camargo is fine but isn't really a #4 which is what the Braves need. If Camargo could play 2nd, short and third and at least be passable as a corner OF, then I think taking a chance on Donaldson makes a lot of sense if his 2018 season presents a value opportunity. I wouldn't pay him 4 years $80M at this point but 3 years $45M might not be a bad gamble, especially if that 3/45 comes without a no trade provision.
If Riley plays all of 2019 in AAA and shows that he's ready, and you are fortunate to have Donaldson return to 2017 Toronto form, then you have the option of trading either between 2019 and 2020.