How long is the handle? 60"?

Yes, though that includes the portion of the handle in the socket.

What steel is used in the head

Given that they are made from used disk harrows, the answer is likely somewhere between 1050 and 1070. Unless they do there own heat treat after welding the socket, the hardness would be around RC 40-45C. This, I believe, is about ideal for a blade that is likely to encounter rock.

Is the handle made of hickory wood?

according to the Prohoe website, this tool is offered only with Ash or fiberglass handles.

How well would this work in gravel drive that is overgrown with tough weeds & crab type grass?

Well it's very rugged and heavy duty, but I don't know how it would stand up to a gravel drive. The stones might destroy the sharp edge. I guess it's up to you if you wanna blow $50 or whatever to see! It is very effective on the weeds in dirt. I cleared a 2'x4' section in about 30 minutes