Mercenaries were the most expensive, for the most part only professional soldiers of the time.
Bodyguards didn't exist. The guy that stood next to the king was the guy that would take the arrow to the knee instead of him. There was no such 'title' as bodyguard, and nobody was given plate armor for being a bodyguard.
What some fantasy writers could enjoy calling 'kings guard' are just semi-professional soldiers that the lord gives equipment to. Normal, common equipment - mail, a helmet, a wooden shield, a spear or a crossbow. The guys that accompanied the king were the guys that happened to be around, for the most part sons of lords.
Anyone that could afford plate armor would get one if he intended to go to battle. For the most part that's lords, their sons, cousins, nephews, grandsons, most of the land-owning knights and who ever were lucky enough to get it off of someones corpse or win it in tournament.
Anyone who walked in plate, actual non-fancy-parade plate, was fit. Because if he had it, and he went to war, he had it on at all times, he practiced with it all the time, and most likely had a lot of a lot of a lot of a lot of a lot of experience in the warfare trade. We aren't talking about some modern pansies dropped off from modern society to a medieval forest, knapping stones together. We are talking grizzled, crazy-eyed, viking-like, veteran bastards that know only how to smash someones scull in and been doing it since they were 8. Maybe they wouldn't win in a competition against modern steroid-ridden athletes, but they would most certainly beat all of them to death, just for fun after a good night of face-smashing in the tavern.
Plate armor is expensive. Fitting it to a person isn't. If you had a plate armor, you were either too rich to care or lucky out of your mind, and you've been using that plate for decades. You know how it moves, it knows how you move, without it you feel naked, too light on your feet, uncomfortable.
