Willem Dafoe is an excellent actor, and especially good for a not-overly-trustworthy character. He's the green goblin in Spiderman and the rogue FBI agent in Boondock Saints (among other things).
I am not sure what age ranges we should have in mind for actors for dwarves. Dwarves typically have a lifespan of 250 years, and Gimli was 139 years old when he was a member of the Fellowship (but 'too young' for the Quest for Erebor 77 years earlier when he was only 62. I would picture Gimli has a middle-aged looking dwarf - not white haired, but not to be played by someone in his 20s, either.
As for height, when you cast a taller, broader-shouldered actor and then use forced perspective to shrink him, it works better for maintaining dwarf build. Short and stocky might be good for hobbits, but isn't necessary for dwarves.