Well 150 bucks is a tight budget for a true robot style compeditor, especially if you plan to scratch build.

My suggestion would be to go to Wally world, choose the cheapest all terrain RC truck and modify to add servo actuated dozer, grabber, dump wired to controller box.

Even the RC controller can be taped together to your custom servo control box, for example.

The battery, since wired, can be located in the controller instead of the vehicle and also supply the battery terminals in the empty battery box in the RC truck, for example too.

If this competition is timed, more reason to go with all terrain tired vehicle instead of tracked.