Okay, today I made a pure I had into a new P2P Pure.

Stats:
50 Range
30 Strength
21 Attack
40 HP

What should I get my range/attack/strength levels to before I start PKing?

Thanks

Also, what quests should I do? I've completed only F2P quests. Vampire Slayer is one.

Well, it depends on your pking style. Since you want range/strength, i will guess you want to range/g maul. I would recommend the following:
~80 range
~70 strength
50 attack
Optional:31 prayer
Optional:70 mage/82 mage (burst/blitz)

As for quests: I would say:
If geting prayer:
Restless ghost
Animal Magnetism
Ghosts Ahoy
Priest in Peril
Mountain Daughter

Otherwise:
Death Plateau
RFD up to mith or addy gloves
DT
Lost city
MM if you want addy gloves
Tree gnome village
Waterfall
Troll Stronghold
Fight Arena
Various others as requirements for other quests

Making a pure is alot of work, dont expect to come in and instantly be able to pk, much training is required.
If you have any questions, pm me on my pure: R4ng3 2h3r2

Hope I helped =]

What's more successful, a pure without prayer, or a pure with prayer? Also, should I think of getting 60 attack rather than 50?

Thanks.

I would say 50 attack until around 90 strength, for the maul has better hits, prayer is very useful in gaining hits without hitpoints, and i say at those levels, you should get it, 44 should come later around 90 range, stay 31 for the time being. Remember, quest all the skills you can, its very useful in preventing unnecessary hitpoints levels. Gl with your new pure.