Who am i suppose to buy?
Well, i got a easy question! who am i suppose to buy, or who is most worth to buy?
,
or
? or just save my IP to 
Well, i got a easy question! who am i suppose to buy, or who is most worth to buy?
,
or
? or just save my IP to 
start with champs who can be used in multiple roles
figure out what role you like most after playing a lot in all slots
expand that role's champions
your picks there are all over the place
What role do you play? What existing champions do you have in your pool?
E.g. You main adc but already have caitlyn, tristana, jinx, vayne, and kog'maw. You second mid but only have Annie and twisted fate. Then buy ahri.
If you main nothing yet and don't have much of a champion pool in general, I'd just outright say buy ahri. She is the most recently discounted champion from 6300 to 4800ip. I bought her the day she was discounted, and she's been one of my mains ever since.
I'd recommend against cass, a lot of players don't seem to like her after the rework. Lucien just received a nerf too, so if you're buying him because he's strong you may want to rethink it (strong picks are constantly changing due to rebalancing).
Irelia is someone I enjoyed when she was on rotation. I've considered buying her too (but I don't play top really, hence not yet).
For general advice, buy a champion you find fun and have tried on rotation. If you buy an "OP" champ they may get nerfed and a meta champ is susceptible to a changing meta. You have a lot more control over what you find fun though :).
Irelia. She is a very fun champion worth a monstrous late game, her only problems are her first few lvls in lane. She is really fun and her kit is refreshing and iterating, with Q reset/refund. The good thing about Irelia is, she has very few counters. Poke lanes always lose to her because of her stun, and all in lanes lose(most of the time) because of her true damage. Also, if ganked, she can Q to an enemy minion to juke the jungler, or she can Q to perhaps jungle camps to make LC$ BIG PLAYS. She is a balanced champion who can deal damage while also having an inherent tankiness.
THANKS EVERYONE! i really appreciate your help, and have a nice day!
It depends on what role you want to play and your already existing champ pools for various roles. You have 3 roles out of those 4 picks. Ahri, Irelia and Lucian are all good and strong...Cass in my opinion isn't that great at the moment and she's very situational.
Lucian is by far the top ADC atm...however, that makes him an opposing primary pick as well as high ban bate...so chances are you won't be playing him a whole lot in ranked or draft unless you're lucky. Ahri and Irelia both have really low ban rates so that makes them a bit more viable to spend IP on in my opinion if you don't just have a bunch to splurge.
I enjoy both champs...but I find top is easier to get into solo lane wise for when I want to play a solo lane champ...so everything being equal and not knowing what you want...I'd have to say Irelia.