I dive in mud holes (we call them quary's) where the water temp can be anything from "cut a hole in it" to mid 70's. (Reason: my wife won't let me move someplace warm yet).
Diving in a semi dry 7mm farmer john and jacket isn't all that much of a problem. However, I am buying a dry suit and it has very little to do with time in the water. It has to do with time out of the water. In the spring and fall you can get fairly cold or hot in a 7mm wet suit when you aren't in the water.
Last fall we did some late year dives, and everything was fine until I had to get out of the wet suit. The changing area wasn't heated, and the wind blowing dropped it a tad bit more. The guys in the dry suits didn't have any problems with it.
Also, on a few deep dives in the mid 40's F, the wet suit did get to be a bit cold. I'm pretty tolerant of cold, but I can see lots of people not being able to do it. I know my wife and daughter can't. Their tolerance is about 55 F in the same make of wet suit.
The main advantage a dry suit gives you is the ability to alter the insulation properties of the suit to match the dive conditions with relative ease, just by adding more or less clothing. Wet suits don't have that advantage.
The disadvantage is the old sailors addage if you get your hands wet you will need to pee soon. Of course there are always dry gloves.
I personally will probably use the dry suit for cold weather and water, and switch to a 3-4-5 for warm water days, and hang onto the 7mm as a backup for when the drysuit has to go in to get fixed, which seems to happen eventually.
But back to the orginal question, depending on your tolerance for cold, you could very well dive in a 7mm. I do suggest getting one that is semi dry. You might try renting one from your LDS and see if you feel comfortable at depth with it. If you don't that will save you a lot of money up front.
Also, one other suggestion, unless you "fit" the average mold that they use for wet suits (I don't) you might want to consider a custom suit. My LDS charges an extra $100 for it, but its worth every penny.