The Mamba Max Pro (MMP) isn't the esc I'd recommend if you were buying new but it can be used. The MMP's Bec is weak so dual servos will be on the slow side. But the main thing is the amount of amps it can handle, the MMP is a 120amp esc and while others run 120amp esc's and say it's fine I personally like some wiggle room. A 120amp is fine if geared within the esc's limits and if you don't want to go much faster than brushed you'll be plenty safe. I'd guesstimate as long as you're under 40mph you'll be fine.
Brushless motors aren't waterproof as water can get inside them, but they do continue running even when submerged. The thing that needs to be waterproof is the esc. But as far as motors go, anything that measures 40mm or greater in diameter and aprrox 74mm or longer with a kv 1800-2200 will be fine on 4s.
I haven't looked into the 2 speeds on the grey chassis trucks that much, but I don't recall hearing anything about them failing. You won't need 2 speeds so just put it in 2nd and leave it there. Usually it's shifting while on the throttle that causes failures in these types of trans. The single speed trans are known for being basically bulletproof as the gears are metal vs plastic.