I did the Edinburgh Spartan Sprint recently, I wore a tight fitting Nike t-shirt and a pair of adidas running tights. Worked fine for me, I'd say avoid anything loose as it will get heavy and flap about. I put an old pair of trainers on that were last worn in a triathlon - had the stretchy laces - this was not good as they kept coming off in the mud. So make sure your laces are fastened tightly! ENJOY. It was great fun.
This is a piece of kit for someone doing intense sport outdoors where water and sweat abound, seems pretty flawed to me that it isn't waterproof / water resistant. I won't be wasting any more of my money on a replacement that is sure to fail in the same way.
I bought mine from Amazon, the manufacturer's description on that page makes no mention about the lack of water resistance.