How rare is it to draft a national team youth player? Does scouting matter or is it luck?
You don't "draft" a U21 or NT player. First, you will almost always have to do some scouting. Second, you would have to be one of the first few teams to pick, or be in a league with few human managers. From time to time people get lucky and end up with a draftee that has good potential. Most of the time, though, scouting is the only surefire way to have a chance of getting them.
If I finish top of my league why should i spend money on the draft if I will have a high pick?
Like I stated above, the only times you want to invest heavily in the draft is if you will finish near the bottom of the league (if you finish in the middle of the pack, moderately investing in the draft isn't so terrible, either). If you do finish at the top of the league, AND there are little to no human managers in your league, go ahead and scout. Chances are, you will get a good draftee.
Why can't you trade or buy other teams draft picks?
You can buy draft picks of other teams (if they decide to list them on the transfer list), but you cannot trade draft picks with other teams. If you could, then it could lead to the draft becoming unfair for some, or in other situations, cheating may ensue.