RE: Traxxas brusless motor?
Yea brushless is the only way to go IMO. Since you have 8.4 and 9.6 batteries already. I would go with the VXL since you want waterproof. Other kits out there for around the same price will run good on that voltage too but I don't think they are waterproof.
If you only had 7.2v packs, I would suggest a kit with a little more kv (rpm's per volt) but the VXL will run good on your packs. You wont be disappointed.