Lol, show him that mobile garden cart, and ask him how he would do one "on the cheap"....then you can tell me.
Here’s what I would use if I wanted a mobile garden cart. It is metal, but light weight. I have two, one holds my welder, so it is stout. You would need to drill some drain holes, but if you can build a green house, you can handle it.
http://www.harborfreight.com/16-inch-x-30-inch-steel-service-cart-5107.html#.UyC1zIUtpw0
The store usually has these in stock.
Or this one for more $$$ but it is plastic.
http://www.northerntool.com/shop/tools/product_200443625_200443625
Like Harbor Freight, they have a store in San Antonio.