Personally, I tend to find that finding your archetype - a certain set of champions that follow the same general rules (melee carries, engage tanks, etc.) - tends to be the easiest way of finding what champions you are going to want to branch out to. For example, after playing Yasuo for an extended amount of time and falling in love with him, I started branching out to other champions of his archetype; Melee Carries. My champion pool and my personal capabilities expanded greatly after finding this out.
Conveniently, cycling through all the 450 ip champions is going to at least expose you to nearly every dominant archetype in the game.
Rotation/Burst Mage - Annie, Ryze (Pure AP)
Ranged Auto Attack Carry - Ashe
Fighter Tank - Garen, Poppy, Warwick
Dynamic Champion - Kayle
Melee Carry - Master Yi
Tanky Mage - Ryze (Mana tank)
Ranged Caster Carry - Sivir
True Support - Soraka
I also can't fit Nunu in here because I have no experience with him, lol.
You're still really early leveled, so I don't think you should be overthinking it yet. Just get your feet wet with as many 450 champions you find, figure out which champion of this list you are most comfortable with, and then buy a 1350 champion who is in the same archetype as your champion. For example, if the champion you are most comfortable with is Garen because you are comfortable being a super tanky damage dealer, you might want to check out Dr. Mundo.
Seriously though. For now, at least get the 450 champions and try them out, and then think about expanding your horizons. Trust me, there is no point in shoehorning yourself into a particular play style yet. Just explore as much as you can.
+1
I used to main ADC. I would destroy bot as Vayne.
Then I left the game, came back and tried to ADC again and was terrible.
I switched over to tanks. I started with Pantheon, he has stun, poke, and his passive was perfect against most top champs.
Then I moved to Darius, Olaf, Garen, Mundo, Trundle, Volibear, Shen and now I just got VI. Hecarim is also one of my favourite champs.
I love bruiser champs.
And I agree on getting the 450 champs. They're a good precursor to the more expensive champs.
And they're really easy to learn and use too.
If you could master Master Yi, you could easily have a good win ratio.
A lot of lower ranked leagues ban him because they don't know how to counter him well.