I have no problem with the Braves spending to address third-base, and I'm actually not too worried about 4-5-year contract for the right player—especially given I don't think the Braves will really be competitive next season, either.
I just don't think Moustakas is that guy. His on-base ability scares me, as does his age (29, when—again—I don't think the Braves will be really competitive until he's in his age-31 season); meanwhile, his power-burst this season will probably mean he gets an extra couple million on his AAV.