The ones with built-in games are most likely not the original hardware.
The exception to this is if it was a real Genesis, and the "built-in games" was just a flash cart with games on it; however, what you found is probably not that.
I know it's probably heresy around here, but not many of the Genesis games stood the test of time. There's always the Sonic series, Streets of Rage, Super Hang-On... but the original hardware might be an expensive proposition for only a handful of great games.
If I were you, I would save my money and keep looking for a Dreamcast.
P.S.: I love the Genesis model 3 controller. That raised D-pad is killer.