Hi Mozi. I'm no expert on anything, but thought I'd offer my perspective on your questions (while bumping it up in the queue!).
(1) There are no guarantees, but generally speaking mixed genders are a bit easier.
(2) Age difference can help for sure initially, but I personally wouldn't get too caught up in worrying about 2 yrs + or -. The real issue is the life stage of the dog. Puppies take lots of time and effort, older dogs not so much. At 3 yrs of age your dog should be fully mature, but still in his prime, and so getting a puppy or even an adolescent will certainly tax your spare time! Both would still require adequate training and exercise.
(3) It's not clear from your comments how you would be able to participate in any dog sports if there are no clubs or trainers in your area to work with. There is really no such thing as a "trained" adolescent dog, started yes. But dog sports of any kind require an ongoing effort and practice. Even basic obedience stuff loses polish if you don't practice frequently, competing in a sport of any kind would require even more time, effort, and equipment than it sounds like you may have at your disposal.
Hope that helps!
It's not the size of the dog in the fight, it's the size of the fight in the dog. Mark Twain