As a triathlete, I would definately recommend a wetsuit that is made for triathlon if you will be swimming in it. As opposed to the diving/surfing types, these are made with smooth neoprene on the outside to help you slide through the water eaiser. It also has a thicker neoprene on the torso for bouyancy, but a thinner one in the arm/shoulder areas for ease of movement. I regularly swim in low-60s water and am comfortable (after you get moving, it's always cold at first). In that temp, you may want to consider booties, gloves, and a neoprene cap as well. Depending on your cold tolerance and body fat, you may or may not need them.
Look for brands like Xterra (the one I use), Blue Seventy, 2XU, or look for one that specifically says "Triathlon" wetsuit.
As for the vaseline, a much better option is something called Body Glide. Ask for it at your local sporting goods store or running store. It looks like a stick of deodorant and applies the same way. Just put it on anywhere there is contact while swimming to avoid chaffiing. Also, a little around the ankles will help get the suit to slide on eaiser.
As for fit, it is not necessary to try one on in a brick and mortar store. Speaking for Xterra, where I bought mine, they have sizing charts online and if you get it and it doesn't fit, they have no proble exchanging it for another size.
Good luck and have fun!