I'm up in MN and the lakes up here are warm enough for short 7mm dives. Heck there are folks up here who ice dive in 7mm. The truly daring can stay down longer, but I'm a wimp -- which is why I own a dry suit.
Honestly, most folks with dry suits will tell you there are two types of temperatures -- those that can use 3mm and those that need dry suits. There really is nothing in between. So because of that common wisdom, I'd advise against buying a 7mm wetsuit. But I'd also advise against buying a dry suit at this time if you have the option of renting a few. They are highly configurable, and you should figure out what features you'll really want before you purchase one.
As for the opinion on cold water diving: I personally love it and highly recommend it. It's a vastly different experience from warm ocean waters, and not everyone enjoys it. But the challenges and sights it offers have a charm all their own.
An early spring dive in a freshwater lake will allow you to see stuff you'll miss in the warmer months when visibility drops and foliage takes over the shallows. It's not as pristine as an ice dive, but is still well worth the time. After all, any day under water is better than a day above it.
Doing a dive this time of year in a wetsuit will be a thermal challenge. If you are prepared for just how cold you may feel (particularly on your face and hands) then go and have fun. Assure your LDS guy that you've been prepped and understand that this is going to feel like jumping into an ice bath (and it does!) but that you just need to be under water!
Make sure you go with folks who have some experience in diving cold water. And be prepared for a few buoyancy challenges as you have to deal with much more weight, and suit compression.
Good luck and tell us all about it when you get back!!