Did you do Slug Menace? Do that if you haven't, it unlocks proselyte. Sir Tiffy Cashien sells it in Falador park after the quest, same guy who sells you initiate. It's a good deal better than Initiate, and still quite cheap. I'd suggest enough runes for at least 200 casts. More as backup is always better, of course, plus you can sell whatever you don't use after. But really, you don't need runes, just brews, super restores, and three range potions. I hear a rune crossbow with mithril or higher bolts is best - you can wear a shield (Zamorak book for prayer + attack), it hits faster, and doesn't degrade. I'd suggest wearing an accumulator if you do decide on a crossbow - do the Animal Magnetism quest.