What should a lg composting bin (green plastic) sit on ? the ground or a "solid" base?

if it is an open barrel type, without an actual bottom, and you put it on the cement, eventually you will get moss growing around the bottom to a certain extent, so in a dirt corner of your yard close to where you will be using the compost would be the best. Don't forget you will have to carry the composted material from the barrel to the area :)