1. Spirit Sword is equip level 35, Mystic Bone Sword is equip level 36, Axe of Torment is equip level 40, Spirit Ripper is equip level 43, Axe of Havoc is equip level 49.
Spirit Sword is obtainable by using
Spirit Crystals [LINK]. Mystic Bone Sword is hard to find and expensive. Use Spirit Sword until 40 and then you Axe of Torment until 49. You can ignore Spirit Ripper.
2. Velanthra Imps is the best place if you can take it. If you can not take it the second best place is Path Imps in Dungeon Server.
3. I do not know for sure. Maybe around level 35?
4. If only leveling do 29 Agility and 20 Constitution and all Strength. You get more HP with Vis Mederi Rings or even Salubris Rings.