Eng, Enstart, and Engoff are the engine files. Remember, they have to be 22khz, 16-bit, Mono for them to work properly, and they *must* be in Windows PCM format.
You can attempt to conform to AHII's rather ancient Startup sequence, which only gives 7.6 seconds until the Engine file is turned on. Or, if you'd like to conform to something called *realism*, you can keep a full Startup sequence, though keep in mind it'll overlap the Engine sound if you throttle up before the startup fades.
As for "best", I'd say record both inside and outside of the car. You won't be able to use both waveforms in the game (read: antiquated sound engine), but you can switch between them, depending on what you like best at the time.