Only buy Tier 3 runes after level 20. I'd recommend picking up a champion you really love and start getting good at him/her. Once you do, ranked will be a lot easier. I would recommend completing your rune pages by level 25 or so, because runes give you bonus stats and give you superiority over your enemy.
Specifically for rune pages, you can read the dude below me, but I do recommend buying more rune pages, as not every champion is best suited for the general AD/AP pages. Azir for example needs CDR glyphs, MPen marks, Per level HP seals and AP quints. He won't function at max capacity with say Mpen Marks, Armor Seals, MR Glyphs and AP Quints. Get what I'm saying?
If you don't heres the short version:
Each champ is unique, and to pull the max potential from them you need unique rune set ups.
Side note: More then one rune page can prepare you for multiple different situations depending on your opponents choice of champion
As for purchasing champions, I personally recommend HIGH skill cap champions. What they force you to do is to practice basic strategies and tactics. For example,
. He is probably one of the hardest champions to learn and do well with, his mechanics are tricky as well and you need REALLY GOOD positioning. You can learn warding and stuff with any champ, but things like positioning and learning when to engage and disengage is great for difficult champs.
I am a little biased since Azir is my favourite champ so far, but Azir really forces you to know when to engage, how to stay in position, how to disengage, how to escape, how to poke and how to go all in. So I recommend Azir as a pick to learn ALL the basic and advanced tactics this game has to offer.