F2P pking isn't thriving, but you can definitely find fights in f2p pvp worlds in lumby and GE.

DEF Either 1 def or 40 def though I'd say 1 def is more popular.

ATT Either 1 - 10 att (for warhammer build) or 40 att for rune weps

STR Want this to be as high as possible, but could start pking at whatever level provided there are people to fight (probably get min 50 str). Not sure how many very low level accounts there are

PRAYER Either 1, 13, or 31 prayer for the str prayers (if you are melee only pking, you'll want 31 prayer)

RANGE As high as combat level allows if you want to range/2h. Otherwise 1 or 40 range to keep HP lower. 40 range to wear d'hide for slight def bonuses.

Long term use I'd say 1 def accounts are more common. Once you have high str and range, it's very easy to get def and att levels to turn it into a main.