This is a fairly complicated project you have taken on, and one I have read into before. There is actually a lot of need for this type of equipment in humanitarian aid as well, and unfortunately most of it cost many thousands of dollars and does not have the capabilities you are looking for. I have seen simpler versions of what you are trying to make, but yours will need to be custom made. The hardware costs will mostly depend on chassis and solar panel capabilities versus maneuverability, as well, ability to carry weight. Weight becomes a concern and it may be worth while to consider having your own metal detector made as well, instead of using a garret (I'm not a big fan) and attempting to interface it with other circuity. I could not afford the hardware to take on this project, but I am interested helping you complete it. I have experience with programing and electrical circuits for micro-controllers, working with different robots, working with metal detectors, and would enjoy the challenges. I would also prefer working by the hour on the project for you as I imagine there will be many little challenges along the way. Good luck on this and I hope you keep me in mind.