Amulet of glory, try to OSRS gold obtain an enrage whenever you are able. Ardougne cape 3, or the soulwars cape. While it sounds interesting but wouldn't it defeat the point of slaying? The fun of learning to Slayer is to learn something new and finish every job, and that's sure it's fine. But isn't it also a way to earn money? I'm sure that eventually, prayers would increase costs to the point of losing money, isn't it?

Thanks for all the suggestions everyone. Even with prayer potions , you are likely to profit from slayer as they are quite affordable. With just the drops you can make a profit, and If you don't, then purchasing Slayer Dart runes from the master of slayers for 35 Slayer points (you receive 36 points for completing two tasks with Kuradel) will cost you around 70k, which is a decent amount and could add up to an amount of money if you invest your money in this instead of blocking or cancelling tasks.

Is it possible to train on bandits already, or should I opt for Giant spiders inside the Stronghold until 70 def and then change to bandits? Sumona needs 85 fight and smoking kills, which you should do when you're a slayer.

Chaeldar requires either 70 or 80 combat (i cannot remember) as well as access to Zanaris. Black Mask is purchased through GE for around 100k. It's required to create an entire Slayer Helm, so don't consider it an unnecessary expense. Purchase the ones that are not charged. I don't pray when slaying because I'm not a very effective slayer, however, others may know more.

Because u're f2p, you cannot range elvarg, the dragon, since f2p's single-handed rng is the weakest crossbow available in game. I would suggest around forty or so atk and 40 str, and 33 def or cheap OSRS GP more prior to attacking it..