inuzuka, what do you want in a car? What do you want it to do, what is your budget for insurance, maintenance, repairs and fuel? Buying a car isn't just about what you can afford right now, but what you can afford tomorrow.
Do you want to have a vehicle that can haul a ton of stuff, corner well, be a comfortable sedan for you and your friends? You can't have a car that does everything, so you need to look at what you want the car to do and what vehicles are best for those needs while still being within your operational and purchase budget.
Domestic cars tend to be cheaper to repair in both labor and parts, especially compared to import luxury/sport sedans like the Infiniti. They are also cheaper to purchase, and with a $30,000 budget you can get a newer model from the near-current generation without much trouble.
I'm going to go out on a limb here and recommend a Mustang V6 with a manual transmission.
https://pic.armedcats.net/b/bl/blind_io/2010/09/17/fords-2009-mustang-took-top-ratings-in-the-sports-car-segment_100223974_m.jpg
It has a big enough boot, it's iconic, relatively cheap, inexpensive to own, with the manual it will be fast enough without enough to get you into trouble, and it looks cool. They are very customizable and there's a ton of aftermarket support, which will be good because you can probably get a very nice example for $5,000-$7,000 under your budget.
EDIT: My cousin has a G35 and the maintenance costs on it are astronomical to the point of eating his budget for a second car. The car needs new rotors about as often as it needs brakes, it eats tires, and is expensive as hell to work on. A new headlight bulb was over $200.