Well from 55-60 rangned i trained at lessers (Safespot) with bronze arrows So at about 55 you should be fine to train there, Plus if you use your iron arrows you will find it alot easier
Just a calculation with the big bones Pray level 45: 61,512 XP Pray level 50: 101,333 XP Need to bury 2,655 Big Bones from 45 - 50 at 15xp Per Big Bone So you would need to kill 2,655 of what ever your killing to get the bones Hope that helped
Yeh definatly collect the Big Bones rather then buy, will save you a fair amount of gold pieces. The runecrafting depends on your need for money back from the skill. Either method should just about take the same time though.