I have rented suits before at beaver divers in avon. They don't have many but they are a LOT better than the farmer john you will get from the kayak shop. They are not tri suits, but they are long sleeve, warm and will be more buoyant. That is your best local option. I used a farmer john (that was to big for me) in the boulder peaks and it was awful. My attitude, like yours was I am not going for speed but believe me you do not want to be swimming with the weight of a bad suit. If you are not a strong swimmer, do the best to get out of the water as fast as possible and save your energy for the fun? stuff.
At the end of the day, I decided kayaking was a heck of a lot more fun that triathlons. All I could think about was how much pain I was in and how much fun my friends were having at gore. Don't mean to get you down - I got my butt kicked (to much boating, to little training) and still feel the pain.
Have fun and good luck. It is an awesome accomplishment and you will feel great when you finish. I have done a few races and live in Vail, so if you have any questions feel free to e-mail me at [email protected]