I have almost enough money to but a 3150 IP Champion, but i dont know who to buy?
Help Please?

Depends on who you want to play. I used Akali during the free week and she was pretty fun. So I suggest you play some cheap characters in the free weeks and then decide.

Xin Zhao is an extremely popular champion since his rework, and he's been quite successful in both the Ranked and Blind Pick brackets. He costs only 3150, and I've heard quite a lot of people saying he's worth much more. Hope that helps =].

Olaf is pretty good yop and jungle. But depends on what you play. If you play mid alot morde isnt a bad pick.

For what?
Mordekaiser can put out punishment like no other and is extremely easy to score pentakills with
Gragas is probably the tankiest ap caster ever
Xin Zhao can take over games by himself if he gets to fed/ too much farm

"best" isn't a thing in league of legends.

The "best" character for 3150 IP? The one you think looks the best

I'm stuck on this question too haha. However, i do have shaco and he's great for jungling. You don't even need to jungle with him he's great at bottom lane too.

Grab a champion that can fulfill multiple roles. Bruisers are common. A bruiser with a stun (or other cc) is great for kill lanes like Pantheon. ANd Pantheon can top or jungle.

If you play mid I would say gragas for sure, although he can be hard to play due to being melee and having pure skill shots. He is so strong right now and is making a rise in top competitive play. Once you know when you can be aggressive in certain match ups mid and how to lane early, he's one of the strongest for sure.