^^^ Check out that thread. Some really good info.
For your budget, I'd suggest picking up a couple cheap manual flashes. Get some manual Yongnuo's and some Yongnuo triggers and you can have a cheap setup. Then from there, experiment. You will know what you need from there as you shoot.
For the lens, that 55-250 will work great. Shoot cars at the 250mm end of the lens and you'll get some great photos. Using wide angle lenses on cars, really makes them seem more bulkier than they are. It's fine for a few shots in the set, but not for an entire shoot. So use the 18-55 sparingly. The 50mm f1.8 would be great for those detailed shots. Shoot wide open to get some nice shallow DOF on the details to draw the attention to whatever you're shooting and you'll do fine.