My scratch builts (not Stegs, but two treemen and a few others) have all started with either (1) plastic tubing and steel wire or (2) polymer clay. I only use these as a base, then I use green stuff to skin them.
I use the tubing and wire to essentially build a skeleton, and then GS on top of that. That is how I did my WE treemen. But they have little musclature, so the tubes were a good base.
However, the Stegadon is a but bulkier with very heavy muscles, so the mere skeleton structure won't be enough and the GS will be prohibitively expensive. Therefore, I'd recommend starting with a polymer clay as a base. (If you haven't used polymer clay, it can be baked in the oven at under 250 degrees.) You can buy colors and skip the GS and paint, but I prefer to just get one color, model it, and then paint it on my own.
There are firmer and softer polymer clays, so you have to pick based on your sculpting experience. Firmer clays, like FIMO, are very hard and much easier to sculpt for beginers, but you have to condition them in your hands ahead of time to make them sculptable, which takes some time. Softer clays, like Sculpey III don't need conditioning, but are very soft and easy to accidentally deform parts you have already sculpted if you aren't careful. PRIMO Sculpey is a nice middle ground.
The polymer doesn't hold detail quite as well as green stuff, so if you want nice scales, you can do it in polymer, but GS will probably be better.
However you decide to do it, good luck! should be a lot of fun. make sure to show us pictures!
