No RC car is really waterproof. None are truly waterproof and still have a risk of frying when wet. For a 54t spur, start with an 11t pinion and gear up if it doesn’t get super hot. The motor could probably use a heatsink and dual fans. Make sure you buy a program card.
The Velineon 3500 motor works terribly for my in my Slash 2wd, it gets hot on 2s and super hot on 3s, I can imagine it will be much worse in a Slash 4x4. Get a 3200kv 3660 motor, and make sure your esc is at least 100 amps such as the max10 sct or Arrma BLX100 off of Jennysrc.com.