That's some excellent advice, guys. At this point it's too late for in depth gameplay tips so I'll keep it to some general stuff I've learned as a tournament DK.
If you get on stream and the commentators end up being close to the TV, bring some earbuds and music just in case. If you're not used to playing a match with commentary, it can be distracting, and you don't want that to affect your play. Especially if they start talking smack about DK as a character, which a lot of commentators will do with any low tier. In my local scene people have learned to respect DK but when I travel it can still happen and it personally bugs me a lot.
Bring water. If you end up making it farther into bracket playing can start to get grueling, especially if you've been crammed in a venue with a bunch of sweaty dudes all day (this might be a Texan problem, but I would imagine it's hot as **** in Australia too). This is doubly true if you encounter slow struggle MUs like Samus, Jiggs, Peach, and ICs; these games will take a lot out of you and if you're not used to pushing your limits like this it will negatively affect your play.
Don't be afraid to ask for a minute or two between games to cool down and think about what's going on. If people start to figure out your strategies and tendencies (which is easier with DK than other characters with more options) it is important that you take that time to critically re-evaluate what improvements you need to make to win the set.
Nearly everyone in my state knows that I play DK now, but if you're not active/notable in your scene you may have the added benefit of people not knowing that about you. If this is the case, call a double blind pick for every match. I can't tell you how frustrating it is to arrive at the set-up, see that the other guy has selected Fox or Marth or whatever, and then once I pick DK he switches to Falco or Sheik or something. He's a dirtbag, but there's nothing you can do about it; you should have called double blind. If you're only going DK all tournament then you can only benefit from a double blind pick; even if your opponent has seen you play DK or otherwise knows he's your main, asking for a double blind may give him the impression that you'll pick someone else and he might switch off of a bad MU for you. And if they don't, then you're not any worse off than if you'd just sat down and picked DK from the start. If nothing else it can stop you from being unduly grimed into a bad MU.
Kind of a minor thing, but on Battlefield, Fountain, Yoshi's and Dreamland, it is always advantageous for you to start in the center of the stage and for your opponent to start on one of the side platforms. A lot of times you can start the match with a free up air on them before the word "Go!" even leaves the screen, which can lead into a gnarly punish on fastfallers or free stage position on floaties.
These images indicate the spawn locations for each of the stage, so memorize them and use that information to your advantage. After stage striking, if your opponent has the port you want, ask him nicely if you can use that port. If he asks you why, just lie to him and say "Oh, I just really like how the <x port color> shield looks" or something like that. This spawn location stuff isn't common knowledge so in my experience they will almost always switch with you. If they refuse, then RPS them for the port you want. Technically you're supposed to settle any port selection disputes before stage striking occurs but this rule is also rarely enforced. If you lose the RPS or they cite that rule at you, ALWAYS ask for a neutral start to eliminate that advantage for them. Yes, this is a grimy trick, but screw them, you're playing DK and you need this help. Don't be afraid to switch ports in between games to maintain your advantage, either. Usually people won't say anything about it, but if they do, whatever, just switch back and be like "oh, I didn't know about that rule" even though YOU TOTALLY DID OMGGGGGGG DECEIT!!! On Battlefield and Dreamland P1 will always start on the bottom, and since these are the most common stages to strike to, try to get port 1 if at all possible to avoid any of this trickery. P2 never starts on the bottom, stay away from it.
As far as a quick gameplay tip that can help a lot, practice grabbing ledge out of a run by doing a short hop single-swing punch turnaround (optional fastfall depending on your height relative to the ledge). This is a fast way for DK to grab ledge out of a run and it will also add a swing to your punch for free, which is nice if you (like me) choose to use partial charge punch somewhat often. If you've been watching my footage then you've probably seen me do this a lot, but at 5:25 after the backthrow
I demonstrate its utility as a fast way for DK to grab ledge when he is far from the edge of the stage and facing forward. Really easy to practice, very effective.
Good luck! No matter what happens, Chandy is proud of you.