Rune pages go on sale a lot. Wait for those sales and buy a couple pages.
In the mean time I would keep testing out free rotation champs and when you find some you like go ahead and buy them. Save IP for runes. If you have a weekend you plan to play a bunch I would also get an ip booster to catch up in runes.
It's rough for someone to catch up in terms of owning all the champions but I would say it's best to focus saving IP for tier 3 runes and to only use rp on champs you absolutely love. 5k rp = about 5-9 champions you would love to play. So keep testing them on free rotations.
(Btw when making a post don't use side arrows or it will mark the line of text as a comment.)