Starter motors, just like the ones for airplanes, are used to start helis, as Skyad said. They are big and torque real well, off of the 12v. The actual mating of the starter to the heli depends on the heli, i know mine uses a 6mm hex, some use a 5mm hex, and i believe some use cones (just like airplane propeller cones).

So you need a starter, a battery for the starter, and a adapter to mate the starter to the heli.

Heli gas tanks work just like car tanks, as you said, can be used inverted. A car tank isn't gravity fed, its just that the pickup is at the bottom, and the exhaust pressure is at the top. Helis have a flexible pickup, called the clunk, which can be at the top (inverted bottom) or the bottom, depending on where gravity puts it. Exhaust pressure still pushes the fuel into the pickup just like in a car.

Heli engines carb is pretty much just like a car carb. Unlike cars, heli carbs can usually completely close stopping the engine. Cars have a limiter so when you hit the brakes you don't kill your engine...

Depending on the radio setup, the reciever and battery should be just pretty much the same a nitro cars. The reciever has a bunch of channels (at least 6 usually) and a battery connection. You can either plug the battery right in to the reciever or through a on/off switch to the reciever. My radio has a on/off that can charge the battery when in the off position so i don't have to unplug anything.