As said before, a 3000+ NIMH with a good charger and you are good to go an hour or more in the grass. I've never sealed one of my tanks and I run my Panther in wet grass and mud, but not standing water with no ill effect. If you do want to run thru puddles, you might consider moving your switches from the bottom.
I highly recommend steel gearboxes of any make. I also run metal Idler/sprocket and swingarms on my Panther for strength, but kept the treads plastic for better performance. Added weight is good up to a point, but a whole new can of worms opens up once you cross a certain line. Track tensioners are also a wonderful thing for rough running since the tracks will stretch out over time.