1. 150-500 (500 maybe too much for smaller tournaments, any cash/key prizepool will attract people) Euros is always a good budget for nicher type of games. Keys can also be a substitute.
2. Open and Invite should be seperate groups for sake of newer players (you're trying to attract and grow the scene)
3. Standard ruleset following one of the leagues, could be more leniant with weapon bans or something of the sort, make it a tad more casual or easy to follow.
not really experienced or knowledgeable on the other two questions so i'll just leave it at that