Tt would be hard to get 70-80 strength in f2p try going to stronghold of secruity and train on spiders should be easy exp. I recommend getting 31 prayer 60 strength+ 40 attack for rune scimitar.
70+ ranged, 40 attack, 70+ strength and 60 mage with 31 pray is a great build for f2p pking. Primarily range/2hing in solo battles, and range/maging if your clanning in multi. Good Luck!
definitely go for as high strength as possible and a given 31 prayer. you already committed prayer, now might aswell just keep going all the way. go to 40 attack and stay there if you're gonna stay f2p.
Stick with 40 attack and raise str/range evenly (e.g. 80 str/range 90 str/range etc). I think 1 prayer is better cause in f2p it's annoying as recharging your prayer every fight at an altar. 1-31 prayer is 4 combat levels or so. You could save those combat levels for str and in the end you end up with just as much hitting power at the same cb level.