1. Depends on your game style and personal preferences. For fast growth, and if you really spend a lot, global revival discount for units lost on Prizes or in battles. If you're mostly F2P, then just save your rubies and you'll know when you really need them.

2. Everyone can send you 2 per day. So, if your Brotherhood has plenty of sketches, you'll get there quickly. However, some are quite rare and it takes a while to get them either from Tinker or in a trade. Those you might want to buy for rubies (only 1 per day!).

3. It is wise to sync your hits with tournaments. Ideally, wear the Prizes down during Prizes and Experience tournaments (best if you have 2 or even 3 at the same time) and then finish them off during a Brawl. Level 20 is easy; they go above 300 and their difficulty is exponential. There is a nice (albeit a bit old) topic here on how the rewards system works for Prizes.