Anyways, Enough about Luigi and Ness. I'll get started on Donkey Kong:
Donkey Kong is your stereotypical Big Character, except he's fast, and his range is unnatural. First of all, Donkey Kong is a ground based fighter, so he's going to stay on the ground for the majority of the match. He plays much better in a defencive style, but unlike MetaKnight, he is still a pain aggressive due to his redicious Ftilt which goes past your Pivot Grabs. However he's still going to be out of his element, so try to force him to be aggressive, it's much easier than dealing with him defencive wise.
It's very hard to get hits on Donkey Kong because his Defencive Ground game is much superior, and you don't have many ways to deal with it. He doesn't have a blind spot unlike Dedede, so you gotta pick your moment. However once you get in, then DK is going to be in trouble as his air game blows. His Bair is good, but thats it. DK is also probably one of the easiest to combo because when he gets hit, he "folds out", making himself bigger. However DK isn't going to spend much time in the Air, so you won't have much chances.
DK Still has some tricks that you going to have to look out for such as his classic Headbutt + Donkey Punch Combo, or his Cargo Spike. They're both pretty easy to see and avoid if you play smart. Also Yoshi's Eggs makes DK have a much harder time to charge his Punch. DK I find rather hard to edgeguard. His UpB seems really hard to get into, plus the initial frames have invincibility! I wouldn't recommend Physically Edgeguarding DK Either. DK also has a pretty tough time, but you still gotta watch his range on his ground moves, or his Bair.
Overall, Donkey Kong is very scary on the ground. He has all 3 of the elements, Range, Speed, and Power. However if you get hits in, and remain in control after you get him into the air, then he should fall apart rather quickly if you don't allow him to set up his guard. I say 4:6