I'm likely going to repeat some stuff that's already been suggested.
If you want a specific gender, then find someone who can tell you the parent's mutations... i.e. male is cinnamon/yelllowsided/pineapple and mom is not same mutation. Females will be dad's color, males will *possibly* be mother's color. No need to pay for DNA sexing that way!
Yes, most of the info from budgies does translate out to conures!
You've got it right with diet! Actually, conures don't "need" seed at all! But it doesn't hurt to at least use seeds as treats! Or provide sprouts as part of the diet!
Treats? Well, whatever the bird likes, honestly. I have Merlin, a green cheek here. He is the least picky about treats or food as far as green cheeks go, apparently! Many struggle feeding theirs a healthy diet and Merlin will eat anything I give him! The only thing I've seen him turn down has been dehydrated carrots. He loves millet, nuts, dried fruit...
Your conure (once you get him or her) might be more picky! So feel free to experiment once you get him or her.
I wouldn't recommend anything smaller, cage wise! The bigger the better! If you have a clumsy baby... well, you're already set up for that, too! You can raise the bottom of the cage so if your new chick falls, he or she wont fall as far!
Let your baby decide if he or she wants to come out immediately or needs time to settle in. Some chicks are confident and are fine being out on day one, others get really nervous and need time to settle in because it's a big change!
Many toys meant for budgies and cockatiels would also work great for a green cheek! Yes to foot toys and cardboard!
Yes to training! Green cheeks have a reputation for being "nippy" birds. Unfortunately, I believe this is due to lack of mental stimulation (aka training) and being naturally curious birds. I do not consider them to be any nippier than any other species when you work with them right!
You can feed cockatiel or conure sized pellets. I feed cockatiel sized pellets because one, I have mostly smaller birds, and two, my largest one is missing half of his lower beak! He *CAN* eat the larger pellets, but it's easier and "less waste" feeding smaller pellets... (as Charlie and Merlin both 'swish' around the pellets in the dish, spilling them all over the floor...
)
You can have at least 3 dishes in the cage. One for water, one for dry food, one for fresh food. If you want, you can get additional small cups to encourage foraging or just to split food up throughout the cage. That, and treats!
(not to store treats in, but to drop treats into)
Food wise... you'll have to experiment! Conures often do like to eat holding their food, but a new baby may struggle trying to balance on one foot and eat at the same time.
Chicks are quite curious (usually) so the more you can expose the little one to new things, the better!
You are probably more prepared than you realize!
The only thing you *MIGHT* not be prepared for? How needy conures actually can be! That would be my biggest warning for you!