Why He’s Perfect: The obvious concern would be that at 45, RDJ is a little too old to be part of a “new generation” of Ghostbusters. That said however, he’s the closest match to Murray’s brand of deadpan comedy we can think of.
In fact, there’s no reason why all the members of the new team have to be the same age, and Downey’s additional years could lend themselves to a world-weary, unwilling, elder-statesman role. It could work!
Essential Scene: RDJ is confronted by Venkman’s ghost, who remarks that he looks “kinda old for a Ghostbuster.”