what size pipe? 

maybe to keep the other bots from robbing you would be to have a truck bed that you drop the pipe into, that has a cover that goes on to protect it. 

the speed controls should work fine. they will work on any channel. some radios have mixing functions, so you could have the throttle stick for forward and reverse, and have left and right commanded by the rudder or aileron control. helis use alot of channel mixing. im sure it is possible to get it to work in this case too. it would make the robot easier to drive. you could make it faster, due to better control. 

for the pipe detection, i would use a metal detector (do they detect all metal? or only magnetic metals? )

maybe you could even use radar or x ray if it is a dense metal, and you want to get really high tech. 

actually grabbing the pipe will be the hard part. depending on how deep it is buried, you could try a bucket, similar to that on a tractor, to scoop where it detects a pipe, and somehow sift the sand out. this is where the truck bed can come in again. dump the sand in the bed, (the bed would have to have screen on the bottom, ) and the sand would fall out and leave the pipe in the robot.