I'm not sure that you can rent a drysuit to try out but if your wife gets cold easily, you might find that she doesn't get into cold water diving even with the drysuit. It just might be best to start with a real warm wet suit in our warmest months. If 12-14 mm of neoprene isn't enough to keep her comfortable in 60+ degree water, the drysuit might just be a waste of money.
pdive:Before you invest in a dry suit or even a thicker wet suit you should rent a suit and take a dive. New England diving is nothing like Caribbean diving. Low viz, cold SIs, tough entries (made tougher by the additional equipment) don't expect to see a lot of picturesque critters. Although a lot of the critters you will run into are quite tasty! That being said diving in New England is better than not diving at all.
Thanks for the responses. A few reasons I was interested in the dry suit class: (1) we've never been shore diving in NE, so it would be nice to do our first dives in the area during an instructional setting and (2) it would give us an opportunity to rent dry suits and try them out. It seems like a good way to get introduced to NE diving.
As for David's idea, do the water temperatures on Cape Ann ever reach the 60s? I was under the impression that the highest they got was the mid 50s.