I can answer all of these very quickly.
Train whatever kind of SF that will benefit your team most (If you like to play look inside, then obviously go with an inside oriented SF. Same thing for if you play Motion or Run and Gun or something, then make an outside oriented SF). If you are not sure what the identity of your team is yet then just make an all around based SF.
As for what their skills should look like for UNT and NT, no one can tell you better than the coaches of the NT and U21NT of the nation of your player. ;) Just shoot them a bbmail and thats the best way to find out. If your just looking for a more of a generalization, then in the forums in a few other threads it says what the general basis of a U21 player should look like. But the nations may very as to what they like.
I think you should probably simply look at getting a trainee that you want help first before anyone can give you more advice. I suggest a SF or Guard because big men tend to be more expensive.
Murray/Harris/MPJ/Grant/Jokic - 2020 NBA Champs