I'm a wuss. Our local quarry is in the 50's at any reasonable depth (and in the high 40's past 45 ft even in summer). I dive it in a 3/2, but only to 20 - 35 ft. If I were to go down below that I'd be buying a drysuit.
My wife turns blue in the pool (78F). Even in Roatan, with 82F water temp, she is cold after the second dive. I don't mean "chilly", I mean COLD!
Why not rent a suit and give it a try? If a 7 mil is enough, buy one. If you are cold in a 7 mil you may want to consider a drysuit, as you want to be comfortable when you dive and it would be a shame to spend the money on a wetsuit only to have to upgrade anyways. There are others here who are manly men and will dive a 2 mil in frigid waters - to each his own, eh?