You should check the infinity guide :D :
http://www.dbocom.com/forum/showthread.php?t=22333
1. Properties have boost and resistance to other properties. Depending on the property it can make you do more or less damage. Stones can get expensive though so it's up to you if it is worth to change.
2. Depends on the level. It is kind of low drop chance for higher level stones which is why it gets more expensive. Up to you again if you would rather buy or farm for them.
3. Check the infinity guide for the crafting. I don't use it and not many people do either. It's not very useful.