Of course, keep a lookout for sales. Paying full price is a rip off.
And other people will just tell you to a. But play ranked, and b. Play what you like.
Most of the champs I listed are easy mode on top of OP.
The thing is, OP champs aren't usually OP forever. Everyone will get nerfed eventually if they are really OP, and new champs are usually OP since they haven't been fully playtested.
Also OP is not the same as FOTM, which changes as the game changes.
My advice would be to test out champs in free weeks to see if you like playing them, and you will in time find your favorite champs, who you can then buy, possibly even waiting until they are -50%, which will in time save you a lot of RP money.