Not sure who to buy.

Lux is probably the best choice. She has (probably) the highest total range in the game, plus she scales really well and she's fun.

Ooh. Neat choices. I don't play any of them much, but I can tell you that, in terms of popularity in my experience, it goes (most played to least): Lux, Brand, Vlad. Lux is more of a ranged sniper, Brand and Vlad both get midrange and deal AOE. Vlad has untargetable frames and a 'safer' fight with his spell vamp, Brand has more straight up damage and an actual stun (albeit a slightly difficult one to land). Do what you like, really, but I hope that helps a bit.

Vlad, imo he is sooo fun, he was my third... and he's freaking badass. Also if you love sustaining in lane and not dying and pissing off your enemies he's a good pick... If you're having trouble against a bunch of vlads, brand is the way to go... I have so much trouble when I play vlad vs brand.

Also since you happen to have 4800 IP/the RP equivalent, Ezreal (I really want Ezreal, he's really fun to play AP), Yorick(really good right now, until they do his rework), and Mordekaiser(second champion I bought, but only 3150 IP) are both other great choices.

Lux since she can also double as a support. The other two cannot.