Some springs are 72 like Ginnie. Others, like Vortex, are a bit cooler. I get cold easily so I usually wear a 5mm or a 3mm with a vest and beanie. I highly recommend investing in a beanie. Even if you don’t want to wear it at first, you can keep it in your BCD pocket and put it on during the dive if you get a bit chilly - the sooner the better.
Also, think about your activity level. If you plan to do a lot of swimming, you can get away with a thinner suit. If you’re less active or planning a longer dive, wear a thicker suit. If I know I’ll be in for much longer than an hour, I wear a 7mm and a beanie.