Yes you can rent a wetsuit. Yes, you wear the wet suit directly over the tri-suit. Look around on the race's home page to see if there is any information about rentals, if not check your local tri stores. A wetsuit is not a necessary purchase if you are unsure about the sport; renting is a more cost effective way to go. Pros: well a wet suit will allow you to keep your body temperature higher, increases buoyancy in the water (especially fresh water in a lake normally salt water helps you to float more), lastly it can increase your speed. Cons: if its a poor fitting wet suit, meaning not sung, it can increase your drag in the water, weighing you down, you can get rub burn from suit if its too loose or too tight in the shoulders, neck, etc... there are tri-specific suits, I use xterra www.xterrawetsuits.com

USAT rule on wet suits  Wetsuits: (cut and pasted from USAT website http://www.usatriathlon.org/pages/1684 )

Each age group participant shall be permitted to wear a wetsuit without penalty in any event sanctioned by USA Triathlon up to and including a water temperature of 78 degrees Fahrenheit. When the water temperature is greater than 78 degrees but less than 84 degrees Fahrenheit, age group participants may wear a wetsuit at their own discretion, provided, however that participants who wears a wetsuit within such temperature range shall not be eligible for prizes or awards. Above 84 degrees, wetsuits are prohibited.

Basically it depends on the race day conditions but lakes can be warm during the summer so you might not need one, but if its early in the spring/summer season you will totally need one.

Best of luck to you, I hope I got all your questions.

I can think of a few "cons", but they generally apply to beginners.

-  If the wetsuit doesn't fit properly, you can get some nasty neck rashes

-  Some people feel that their breathing is restricted before they get used to the tight fit.  Not necessarily a con, but it would be a problem if you don't swim in it a few times before the race to get used to the feeling.

-  I've also watched some folks in transition have a tought time getting out of the wetsuit if it's too snug around the ankles and wrists.  Again, just something to be aware of and practice.

You mentioned that it's your first Oly, but I don't know your experience with shorter distances or the expected water temp.  Wetsuits are "nice to have" if the water is reasonably warm, but certainly not necessary.