if you're content with making it a f2p-only pure, they stay 1 prayer. it's better off that way combat level-wise, and it's less questing you need to do. you can, of course, always quest it later on for prayer should you want it or want to be P2P-based.
to address the other quests:
-Death Plateau (Climbing Boots for training, requirement for DT should you get P2P-based)
-Vampire Slayer (not required for anything but it gets you 1-20 Attack)
-Waterfall Quest (Attack and Strength XP, requirement for DT later should you get P2P-based)
nothing else really sticks out to me for a F2P pure that is 40 Attack. obviously, if you choose to go P2P there is a plethora of quests you would need. you can find a full list of those on most runescape fan sites or pure clan sites; MM is particularly known for having full lists of pure guides in great detail.
best of luck with it, have fun