I think you are definitely right as far as weight and cost when compared to the metro drivetrain. A metro would be dirt cheap, since you can find a complete running car for $350-1200, but there are several reasons for wanting to go with a diesel.
One is due to the fact that we drive a chevy diesel 2500HD crew cab truck, so we tend to keep a small amount of diesel on hand at home and more importantly we usually bring a supply with us to our acreage in the White Mountains of AZ, where the cart will get a LOT of use. We have 65 acres, surrounded by a couple thousand acres of BLM and State land, plus many more thousands of acres of roads and washes to explore. We also use a community well right now, until we can get our own well drilled, so that is about 6 miles one-way to haul water, so rather than only one person pulling a trailer behind a quad, or loading up in the truck we can use the golf cart. The nearest store is also some miles away, and I hate driving the big truck to the store. I tend to ride a quad, which means I have to go alone. That also limits what I can carry back to the property with me. The cart will allow me to carry people (if desired), plus carry a fair amount of goodies with me from the store. It will probably have limited use here in the city though, but who knows it may turn out to be my primary mode of transport in the end.
When a small group of us just want to take a cruise around the mountain property we sit or stand on a trailer, towed behind a quad, and to be honest that isn't the most convenient, comfortable, or safe method of travel, especially for the smaller children (5-11), plus the passengers on the trailer get exhaust blown in their faces.
Another reason is for the additional torque provided by the diesel, since I would like to convert to 4 wheel drive, and have enough power to get through sand, mud, water crossings, hills, etc.. I think the Metro engine will have a bit harder time with that, and be more work to convert.
The biggest reason is probably WVO, which I should have mentioned in my OP. I would really like to be able to run WVO at some point, to save a bundle of money. I have friends that run pumping services, and service restaurants to remove their waste veggie oil, and they would be happy to give it to me rather than having more work (and $) to dispose of it. I will then process it here, and run it in my other projects. I'm converting our generators over to diesel for that very purpose. I'm hoping to have practically free fuel when all is said and done.
The engine does not necessarily have to be TDi, non turbo is fine.
Thank You for the input. I appreciate it.