The advise that you get a dry suit from the get-go is good advise. A tri-lam suit with minimal undergarment will do you fine in 68 degree water, and when you add a good undergarment will serve in water as cold as it gets.
With respect to wet suits: I issue a very detailed equipment list before each Research Diver Course. Each student needs to purchase and show up with that gear at the first meeting.
The suits we used, as I’ve mentioned elsewhere, are 5mm, skin two side (now lycra-out), Rubatex GN-231N, attached hood, farmer johns, no zippers. I send my list to all the shops in the area and freely distribute whatever they send back to the students (this is a big deal, twenty full sets of gear with no selling or inventory required: take the order, take the money, deliver in two weeks).
One student did not go to an local shop, but rather to a shop near her home, about a hundred miles away. I got a call from the Instructor in the shop informing me, in a fairly emphatic tone, that, “No diver could possibly wear this suit. They could not put it on without a zipper.” Now, please understand, that I’ve been diving this suit design since the mid sixties, and the only people who need an inverted half zipper in the jacket are incredibly curvaceous women that are of petite statue. This woman was just shy of six foot and her blueprint could have been confused with a plan for a javelin.
Having nothing better to do (and considering that the woman in question was one of the brighter marine geology grad students), I drove up to the shop later in the day. I brought my suit with me. I showed the Instructor how easy it was to put on and take off, etc. We solved the problem, but the bottom line was that this Instructor, well meaning as she was, had not yet worn out here first suit and was just repeating what her Instructor’s had told her. It wasn’t a marketing issue, the shop could and did supply the gear (and nicely matched the prices of the local shops that had sent fliers).
When it comes to dive gear, real information is hard to come by. Most of the opinions that you see on the net are biased either by being the only piece that class of gear that a newish diver has ever used, or was a loaner that an expert tried out on one or two dives. What you need to do is find an expert who is doing the kind of diving that you plan on doing and ask him or her about the gear. That may well not be an Instructor. Don’t be afraid to bore on in, why … why … why. If you do not get answers that you understand, find another expert. Make sure that the advice makes sense in terms that you understand. When it does, buy the gear and never look back.
In other words, drop the DSO at your future graduate school a line.