off hand im thinking a HPI wheely king. $200 rtr. Waterproof electronics. 4wd. comes with a battery good for 15min of runtime.
5 pounds will weigh it down some, but you can put on heavier springs.
The wheely king is meant to pop wheelies, but putting most of the weight forward will help.
It can be a top heavy truck, but you can set the suspension lower to help correct that.
You can also put on a much smaller motor pinion...this will lower the top speed and extend runtimes. stock speed is around 20mph.
Another option is an exceed mad torque. this is a 1/8 scale rock crawler. It has a better center of gravity (more stable) and it goes slow...5mph. It can run for an hour or more on a 5000mah battery. And it shouldn't be hard to setup the shocks to handle 5 pounds. I have a heavy 1/6 scale hummer body on my mad torque and it does well. This is also a 4wd truck with large tires.
Any RC on salty sand will need some work after being run. salt will rust any metal parts. removing the salt/sand is needed.
Exceed also has an even larger 1/5 scale rock crawler and they also have a 6 wheeled version of the mad torque. these trucks cost more, but should be even more stable and handle the added weight even more.
The exceed mad torque trucks (4x4 or 6x6) don't have waterproof electronics. but the electronics sit high in the chassis so as long as you don't go full speed threw a deep puddle you will be fine. and it is possible to waterproof the electronics with plasti-dip.
Most 2.4ghz radio system are good for a long range (I know my trucks can drive far enough away I can hardly see them) how much range were you needing?
Edit: you can also check on the HPI Crawler king. this version of the same truck has a higher turn motor (more torque, less top speed) as well as a longer wheel base.